Early on I will research mass drivers and give/trade/sell the tech to the AI so most of them will then go MD and I generally go Beam early to late midgame.

Once the AIs adapt to my beam weapons I will then switch Missiles. Some games I've switched weapons/defense as many as 5 time to keep that edge over the AI.

I dislike mass drivers because once you have the nano ripper you have to go up to nearly neutrino bullets for something that does as much damage.

Usually I go for beam weapons, but I've recently been using Mass Drivers in one of my games. Although Mass Drivers may feel pretty weak early on in game, by the time you reach the Black Hole Gun, you'll be putting your enemies in a world of hurt.

I've just managed to flip a war with the Altarians using Black Hole Guns, after designing a huge hulled ship decked out 100+ points worth of Mass Drivers, capable of moving 17 spaces in one turn, and with a high missile defence (they are using missiles). Although I can only fleet together 5 or 6 of them (I have 49 logistics points) they can take out about 2 fleets of 10+ Altarian ships before one of my ships gets destroyed.

If you want to Know exactly what your neighbours are building, adjust your espionage slider and when you reach med. espionage for that race, all you have to do is click on the neighbours ship and you will be shown exactly what offensive and defensive weapons they are using. At higher espionage levels you can click on the planet and look at all ships in orbit and how close to completion the next ship is. Hope this helps.

Beam if it looks like most of the AI is not using beam. Otherwise mass driver.

I usually go for a good, tech heavy civ and get medium ships (I call them destroyers) ASAP and large (cruisers) as soon as I can afterwards. Load them up with some of the good defense techs matched to whoever needs exterminating (but only in a good way ) and I can wipe out fleets while taking virtually no damage. Those little one shot kill fighters with no defense just don't last long when they can't even touch my cruisers or battleships.

I use weapons that my enemy is weak in.

I'll use anyone i consider good in each game. Most of the time is between Beam and Missile but in my last game i went for Mass Drivers just because. I usually research an early weapon theory (Laser usually) just to build some basic defense. Then wait to see what others start using and/or research in their attack&defense, and from that information start research what i need.

I use huge ships a lot to end the games. Nothing like big ships that give me lots of space for lots of engines (the most important part) and some poweful weapons and defenses.

Mostly beam, because it's the most sci-fi .

I usually see what kind of defences the AIs have developed, then do a crash program of either beam or mass-driver. If it's a toss-up then usually beam for the psy-laser goodness.