TParis: Humm, I have Civ IV lying around here would like to play. Buying Civ III came to my mind aswell. Which part in the series is the best and why?
Civ 1 is the best, because for it's time it was two generations ahead of everything else and it started the series.
Civ 2 is also the best, because it had multi player and wonder videos and funny advisors and had everything that makes Civ great.
Civ 3 is certainly the best, because it contained so many nice features including lots of nice micromanagement and tons of modifications. I still play it today.
Civ 4 is undoubtly the best, because there never ever before was such a high degree of sophistication in the game. Promotions, great citizens, religion, ... you name it, Civ 4 had it.
Civ 5 will be the best, because it simply looks and feels the best and is easier to use than Civ 4 but with appropriate extensions will be as deep as Civ 4.
So, all in all all of Civ is really great, only Civ 1-3 nowadays look a little old fashioned with an awkward user interface - I really don't know how I managed to enjoy the playing experience then.
So, I would recommend you Civ 3 or Civ 2 if you are reaaaaaaaaaaaaaaaaally into Civ, otherwise rather go to Civ 5.
P.S.:
Anyway, everybody knows, that Alpha Centauri (SMAX) was and will be the best Civ ever.