Hi!

Besides that needed level-2 techs for better abilities, factories, labs... most of my research path fully depends on the situation I'm facing in the game. For DA game it's uually like (in approximate development order):

• Ion engine for almost everything, and a bit lately tech for Impulse for +1 speed.
  
• the bigger the universe, the more the Sensors 1 tech is important, to quickly take those anomalies,
  
• the more habitable planets available, the more the better pop growth is important, so I research Xeno Medicine for quite cheap early boost in PGR, and for the same reason the Xeno Entertainment, that also gives some more money.
  
• cheap parts of planetary invasion branch, if there's a close AI that's not militarized yet. Quite often my first victim in small games falls to just a fast armed cargo hull, that clears orbits of colonizers and scouts, and a few troop transports.
  
• trade branch, if I need better relations fast (big bullies close), or there's so few planets that money from trade routes would actually matter.
  
• some diplomacy if I need better relations fast (big bullies close), and to easier bribe those bullies to go to war with someone else. I used to build Diplomatic translators in older games, but recently, with tech-trade mostly "off", I noticed I acutally need just better diplo than bullies, since having better diplo than Humans and most other good AIs is possible only in late game. The bonuses from researching to Trade and to Republic are usually enough.
  
• better tech for planetary infrastructure, somewhat depending on the planet-development model I'm using (most/all-labs, most/all-factories, mixed), and my financial situation (if I can afford the upgrade).
  
• the more planets are available, the more important morale "wonders" are, for cheap boost in taxes and/or pop growth, Harmony Crystals being the first and the best one.
  
• the bigger the universe, the more Eyes of the universe seem valuable. In some small games (~5 planets per player) I never researched past Sensors I. No place on planets to put there the building...
  
• first real milestone: decide which AI will be my victim in serious war, and what tech I'll need for it. So for the warfare in DA game I need medium hull with ~3 points of attack (usually single Harpoon), enough defenses on it to block attack from the average fleet my opponent is fielding, and one Impulse III engine. Also some cheap miniaturization and logistic for 2 mediums in a fleet, and Warp engines for +1 speed before fighting starts. Most of the Planetary invasion branch, to boost troops already in transports, before invading starts. Produce everything needed, put into position to attack AI's economy: planets and resouce-mining starbases.
  
• Aphrodisiacs to bost growth on newly-gained planets, and to improve growth of troops on old planets. Second growth-boosting tech.
  
• more "money" techs - new planets usually don't pay for themself. So Democracy, Banks or Stock markets, and more production tech to boost econ and morale resources.
  
• more tech for planetary infrastructure,
  
• second milestone: next AI to conquer. Better/other type of defenses, more space on ships - either more miniaturization or Large hull.
  
• next milestone: next AI.
  
• last milestone: decide with what victory type I'll end the game.

BR, Iztok

Challenging? I think you mean higher levels. Today, I won my Terran game on Challenging. Conquering other races has never resulted in the remaining races attacking me, I never had to stop research for military production, never lost a ship, and never got out-teched. My first war with the 1-planet Thalans was a cakewalk, and minor races didn't put up what could be called a resistance. Only the Drengin and Yor fought prolonged wars against me because they had tough soldiers, and were either on the other side of the galaxy(Drengin) or had Super Isolationist(Yor).

All the remaining races got Close after the fall of the Yor, I formed Alliances, and abruptly won a Diplomacy Victory with the Torians nearly outscoring me. I'll continue the game from the last save and aim for Technology Victory(which will take several more years).

I don't know about higher levels or lower levels though, or games with many planets. Maybe the AI works better when they have more than 2 planets(the norm in my Rare-planet games). Otherwise, when due to events or war, if the Bright-level AI gets stuck with only one planet, they usually become largely inept.

For the bigger maps, leave the AI with one planet as well. This lets you avoid the negative you would get for genocide.

Does anyone know whether you take a diplo hit when an AI you were at war with surrenders? And what about those rare cases when an opponent surrenders to *you* even though you're at war?

I'm not so sure. An AI recently surrendered to me (in a war) and I got a kill sticker for it on the victory conditions site.

Lol. I think he means that the "You have to destroy X-number of civilizations" message under Conquest Victory in the Victory Screen drops that X-number by 1 when a civilization surrender.