Space Empires 3. Because you can cut yourself off from all the other star systems in the galaxy by closing the wormholes between them, then devote 10 years to buildinga fleet of cloaked robot kamikaze ships armed with system destroying sun exploder bombs, then unleash them upon the rest of civilization.
Space Empires IV is quite good too. Also has a mountain of mods to extend the playing experience.
5 is... okay. It's not bad at all, though I find the AI in vanilla to be a fair bit incompetent.
Mods can improve the experience, the Balance Mod in particular.
I recommend Sword of The Stars and all of the expansion packs; not sure about the second game. Aurora is another good one, though as many have said, there are bugs and glitches galore.
I get mixed oppinions.
Some believe it is a shadow of how great the first is even in its current state. Others say it finally caught up with its potential.
Eehh. It's not a bad game at all, but Prime still does have an edge on it. I think by now it needs polish, and I think that's what they're doing now that it has been given the 'all-clear'.
Sort of a time thing. They've drastically improved the game in one year, and I suspect that the next year will see further improvement.
If there's another expansion, I suspect that's when the comparison battle between Prime and 2 will really heat up.
They're really generous though. An entire free expansion.