As a long time 4x / TBS gamer, I'm not sure I'd go so far as 'best' but I think you can make a really strong case for in the top three.  I'm new to Galactic Civ and I found my first 20 turns of my first game to be "meh", but I played a little longer and sometime after the 30 turn mark, the "one more click/one more turn" monster got a hold of me and hasn't let go since!

I really like the design choices and direction the developers have picked for the game!  The one area imo that could use some review would likely be the battle camera, as after watching two or three battles and not really being able to catch the action i just auto-resolve now.

I'll have to go with 2nd best...Sorry Age of Wonders 3 is just that good.  But until a 4X TBS Space game comes out with the Awesomeness of GS3's strategic game plus a true Tactical combat element   Then GS3 will be the best 4x Space TBS game.