I started with Civ Rev at Spring, 2010. I loved it. It's not a complex game, but I had lots of fun. At Summer, I decided to get Civ4 Complete. Loved it as well, but it was very difficult (that's why I play vanilla Civ4 than BtS). And then, at Autumn, bought Civ5. Again, I love it, but there's things yet to improve (it should have been released at 2011 or 2012, not before).
Yeah... That was more or less my experience in the Civ series.