I'd say go with moo2 and GalCiv
GalCiv can get a bit repetative, but there is an expansion, and a good bit of mods out there if you get bored, and it still has one of the better political systems, plus cool features like multiple minor races (that can do everything a major can, except win) the combat is a little shallow, the most fun part is probably having to take care of your economy, relations, and morality, all while beating (or bribing) off the AI, its one of those games, where war is some times a VERY bad idea, and some times your only choice.
Moo2 is of course a classic and has some nice features GalCiv should have had, but does'nt (planetary bombardment, ship design, a real weapons system, etc)
I would totally avoid moo3 unless its super-cheap, as its probably not worth the money, and you would have more fun with some game like Spaceward ho! or Stars! which you can get for free (with some digging for registration codes and the like
)
There is one more little known game you might want to consider, which I have bought and played, and is really quite good, its called Space Empires 4, and is some thing like the anti-galciv, focusing more on a simulation, tons of technology, ship designs, special weapons, totally modable, etc though it does tend to have a more mechanical feel to it (it lacks all the nice racial pictures, random event, Galactic news net, etc that adds to the atmosphere) if you want more of a hard space 4x wargame, then thats the way to go.
http://www.malfador.com/se4.html