Admittedly, I haven't played very many turn-based strategy games, but I fell in love with Alpha Centauri. It's pretty old - not sure what you're looking for - and you can pick it up on Ebay for next to nothing.

It's the same idea as the Civilization series, but set on a distant planet in space. I think the end of one of the Civ games has you sending a ship into space to find and inhabit a livable planet. The Alpha Centauri loosely picks up the story from there with the various factions landing on the planet separately and trying to eek out an existence with their ideals intact.

I thoroughly enjoyed the game way back in the 90's and I still stick the disc in every now and then. It's definitely dated, but still a lot of fun.