best difficulty

There is no way to tell as player skills differ, but I would start with "Challanging" for your purpose and give it a try. It also may help you to have a look into the overview on difficulty levels at the wiki:

https://www.galciv.wikia.com/wiki/Difficulty_level

yeah, start with Challenging, see if that works for you; if that is still not enough, go to Tough.  After that go to Crippling.  There is a big jump from Crippling to Masochistic (even though it follows in progression) so you really have to classify the last three highest difficulties in a class by themselves.  Start with Maso, and if you can beat that, you can usually beat Suicidal.

I'm a casual player myself, not into the seriously difficult 'mind everything or you will FAIL' difficulty levels. However, I found challenging to be a bit misleading. It really wasn't. Tough plays pretty well. The AI aren't dolts, they get a little aggressive. They also play some decent battle strategy. I will probably go to crippling soon to see if it's markedly different, just to find the sweet spot between challenge and pain in the arse... 

If you're new to the game, Run through a few on normal to get a feel for the late game. It changes quite a bit once you're out of the colony rush and everyone settles in. It'll also help you to determine your 'tedium level'.  That lets you know what size galaxy to play. If you have a low tedium threshold, a really big galaxy will drive you to tears as you spend turn after turn micro managing the snot out of everything and waiting for all the ships to move... After that go to challenging and then to tough after a few games.

If you've been playing awhile go right to tough. It's a little more 'challenging' than challenging, but it's a nice balance. You'll have to watch tech and planet builds, but you can expiriment and use differing combinations without getting hammered for it. The AI are mostly competent; they will give you some surprises, but they aren't yet to the suicidal rampage stage so you actually have time to grow and develop. 

T

Beating Maso is pretty easy once you learn certain tricks of the game. In fact, it doesn't even require doing anything special, as you can pretty much build basic stuff on evey planet and have the sliders set on 33/33/33 and clean up on the AI. The next difficulty level up is much tougher though.

Although overall, as far as AI plays, ToA is the worst AI yet. DA and DL Ai was much better. I find the AI often plays absolutely horribly on any difficulty level due to certain game-breaking AI flaws that seemed to appear in ToA. These include but are not limited to:

Certain AI's do not expand, noticebly being evil AI's, among others (not all the time, but seems to be the trend). I often find the AI expands to a point, then starts hording colony ships on their planets without sending them out. Then out of the blue, some AI's will send all those colony ships out in a swarm, while some never do. Torians and Terran seem to be the best at expansion. Yor and Drengin seem to be horrible at it. Other civ's usually do a descent job.

AI does not build improvements on many planets at all, usually on immense or large maps, and usually on planets that are far from their initial starting positions. This makes higher difficulties much much much easier than what they should be.

Reports (although I have not seen this happen yet) of AI's not building ships. I have seen the AI build ships with no weapons or defenses even when they have the tech's (usually in early game, have not seen this happen mid/late game).

I have noticed that the AI, throughout the game, and especially in late game, tends to turn off all development completely (same as putting your production slider down to 0), they quit building ships, quit researching, and quit developing social improvements, and it's not that they are broke or in debt. The Drengin did this in my last game 4 times, each for at least 30-40 turns per time, while still bringing in thousands of BC per turn. Torians did it last game a couple times, other civ's I did not check though, although they probably did as well.

AI, or certain AI's, seem to build only factories and morale improvements on harder difficulties (on most planets that actually build improvements), but that is probably due to the fact that they have such a huge economic boost on harder difficulties they still reap in the cash with no econ buildings, and make money on each planet even though they have a factory on evey planet tile.

I'm sure some of these existed in DA and DL, but they were not near as noticeable as they are in ToA. Since the 2.02 version is likely to be the last major update, it's very unlikely any of this will ever be corrected though.

For all strategy games, not just GalCiv, but I don't understand why they did not program the AI to actually protect their starbases and their transports. Makes no sense at all. I have yet, EVER, to even have an enemy transport attempt to attack one of my planets (except in the case where a civ surrended all their planets to me, and an enemy AI attacked one of those planets). The reason is, they just float in their unprotected transports all over the place, and even in late game, a 1 weapon 5 hp star fury can just clean up on em. At least program the AI to send a powerful group of ships along with it. The AI often makes 'Escort Ships' which are usually comparable to Fighters or Heavy Fighters, but weaker. Yet these escorts never escort a transport. Sometimes a starbase may have 3 all attack, but I have yet to see one more powerful than that. In fact, I will bring a fleet in to destroy a starbase, and the AI wont defend it.

The AI doesn't seem to do much of a 'Power Check' in terms of whether it's forces can destroy yours or not (as Civ III or Civ IV forces would do). Just pull up a Battle Axe with e.g. 1 attack and 12 defense to a some of those huge stacks of fleets that the AI sometimes accumulates where each fleet has maybe 3 attack. The AI will attack your ship with every single last ship and lose them all, in the end, your Battle Axe ends up at level 20. I once had a small hulled ship go from 12 hp to near 60 hp from such an encounter in 1 turn.

The AI does do ok, but usually only if they are much more powerful in terms of ships and technology.

Is there an official statement you could point me to? That would be a crime, TA has great potential and would deserve to be tweaked to it's best. It is still a long road to GalCiv3.

Right now I am playing my first game on tough in TA. The Torians gave me a tough fight, they declared war on me and had some decent fleets around. They colonized a lot of planets and build up a strong economy. On the other hand they didn't concentrate forces to defend against my spore ships, seems the A.I. is not aware. Further on if enemy fleets set up a waypoint and find some undefended spore ship or transport on their way, they keep following their path and don't take the opportunity.

The Drengin were running in some kamikaze wars without expanding (and building up before) and quickly vanished. The Korx did expand very well, they took over some minors and finally killed the Thalans. Some others died and now late game there are three fractions left. My Korath, the mighty Korx and the Terrans. The Terrans, although in several wars before, did not build any ship until lately and now they have some lousy fighters without defense and minimal attack that don't even show up in the military display.

In that game I took random enemies, but I would like to hear which races are known to perform best.

2.0 was supposed to be the last update, apart from bugfixes to game-enders.  Brad has stated a couple of times that he just can't stop working on GC2, though, so 2.02 may not quite be the end of the line, either.

In any case, we are well past the point where any significant functionality will be added, and if it's not specifically a "bug", I wouldn't expect it to be fixed, either.  (A good example of something that is a bug is the instance where most evil races don't expand, etc.)

Okay, thank you. So there is still hope and I wouldn't be too surprised if Brad continued polishing TA before he calls his impressing masterpiece finished.

Well, TA already adds a lot of significant functionality and variety, especially if you skipped DA like I did. It just needs some more A.I. tweaks to adopt to the changes. Nevertheless I propably will give DA a try in my next game to see how much better the A.I. acts there like mentioned above.

I'm playing GC II in Campaign Mode, and have gotten through thrashing the Korath (I'm playing as the Terrans) using a pretty darn boring *research/colonize/resource-bank* semi-turtle strategy (because I've never played in Campaign mode, I'm chicken and started in Cakewalk difficulty).  However, I've run into the first flaws in that strategy against the Yor (next up after the Korath), as the Yor don't seem even remotely content to let me turtle; they have sent fleet after fleet (albeit consisting mostly of fighters) to pound and harass my two colonies (forcing me to have my Arnorian Battle Cruiser shuttling between the two colonies while trying to build up groups of Phoenix fighters and hoping the Yor at some point get tired of sending fleet after fleet to harry me so I can actually commence mining the resource areas near the colony worlds).

One thing I DID pick up (that I did not know before) is that all research from the first round does *carry over* into the next; so I may actually move my breakout later in the first round to give myself some sort of parity with the Yor in the second (while I maxed out my defensive research almost completely against the Korath, I did little offensive weapons research, and none at all on ship-building or fleet management, which is likely why the Yor are finding me easy-pickings).

Like all other titles in the GC series, research is the rule (not the exception) to winning the game; the further down you research all the techs you can, the closer (or easier) you'll be to finding your path to victory (or conquest).  Research is not cheap, so trade is VERY important (so mining is just as critical, and so are survey ships), so you may actually want to have more (not fewer) ships out there surveying (however, don't ignore the Eyes of the Universe; this technology, which lets ALL your ship types survey, is actually available right off, and should be researched as soon as possible, and on whatever planet makes it available, especially if it is a good distance away from possible Korath assault).

If you're speaking of techs researched in one campaign mission being auto-researched in the next; I have never seen this behavior in GC2, and would not expect to, although it would increase the realism, and has been, er, "requested".

The brains on this forum, myself included, have determined that trade is not worthwhile for its cost, with the exceptions that you are playing on a tiny or small map.  It is still useful as a diplomatic tool, however-which is all I personally ever use it for.

Eyes increases your sensor range.  It does not let all of your ships survey.  You're thinking of the Galactic Guide Book, which reduces the cost (but not the size) of the survey module to zero, allowing you to add it to any ship design that still has space.  From my perspective, it would be better if it reduced the space of it to zero, as then it would not necessitate taking off an engine/weapon/etc from the ship in question to fit it.  GGB is deemed useless by most and praised by a select few; tetleytea is among the latter.

If memory serves, it actually reduced the size to zero in DL/DA, but in DL, where anomalies don't regenerate, it's worthless as well, and even in DA it doesn't see much use in my games.