Civilization 4 with the Beyond the Swords expansion pack is excellent value and one of the best TBS games out there of all time.
But it's Galactic Civilizations 2 that is amazing beyond belief and which I'd most highly recommend! A second expansion pack having recently been released for the game as well. Picture if you will a scenario where your galactic empire is slowly being pushed back by sheer numbers, seperated from you allies by dozens of star systems and seemingly without hope when on a desolate remove planet your scientists discover a strange alien technology. You immediately throw everything you've got into scientific research, forsaking even military as planet by planet your empire is crushed. And then comes the day you reveal it, a new mothership class battleship twice as large as anything else in the galaxy and custom designed with shields and weapons to counter the enemy fleets. It is sent forth and destroys the enemy before it, a juggernaut holding back the storm as you finally urge your military back into full production and soon are fielding more than a dozen of these massive machines. The enemy adapts, produces new ships with new weapon systems to counter your motherships defences, but it's too little too late and soon they are sending emmisaries that beg for peace with you. You think you've won, that this game is yours, but that is when you receive messages from your distant allies asking for aid against some new, greater evil. With trepidation you turn your fleet to face this strange and new enemy...
Sounds awesome no? That was a desciption of one of my matches in the GalCiv2. One which is still active. The game is amazing. A bit complex at first but once you get the hang of it there's little that's even close.