I personally like Civ IV the best, it has amazing mods. Like Fall from Heaven II...or the one I use, based off of it...Master of Mana. I love fantasy stuff, so having a fantasy overhaul mod is great. Skip to the bottom for the end of my...rant or whatever it is.
Civ V is good too, though. I have Civ V + Gods and Kings. I like how there isn't ultra stacks of doom...but the modding seems a lot weaker, even with non-fantasy stuff. I can barely find any overhaul mods for it on Steam workshop, in fact there is only one listed and it isn't for Gods and Kings. And even the custom maps don't show up properly in-game.
Having said that, I got Civ 3 (which I don't like at all, but it was only 3 dollars)...+ Civ IV and all expansions + Civ V and all DLC (not Gods and Kings), for 20 or 25 dollars on the Steam Summer sale. And I was gifted Gods and Kings for my birthday.
That summer sale though was awesome and was what I was waiting for. I think it totaled 140 or 150 dollars or something, and it was on sale for 20 or 25 dollars. Plus the Civ V expansion gift. I probably wouldn't have got any of it otherwise...except maybe Civ IV, but even then I was waiting for it to go on sale anyway. Since it goes on sale quite often.
TLDR-
In any case, if you aren't sure...can always wait for a sale on Steam. Personally, I only like Civ V because my friend and I play multiplayer, I never play it otherwise. I like Civ IV + Master of Mana, a lot more and have put a lot more hours into it. And even if you aren't into fantasy, there is so many other mods for Civ IV. Civ V really lacks any modding, I'm not sure if it is the modding tools that are lacking...or it just doesn't have a big modding scene...or both.