If I go with success as persistent mass popularity... it pains me to say it here, but the obvious answer is Civ5; it's a 15 year old game that still has tens of thousands of daily players. In 2036 there will still be thousands of people playing it every day. I'd compare it a bit to the interia of Age Of Empires 2; Microsoft has had more success with rereleases of AOE2 than making new AOE games. I would not be surprised to see Firaxis make a "Civ V Definitive Edition" for Civ5's 20th anniversary, rather than develop a Civ8.
If you reject success as commercial success / popularity of the masses, and want something purer, you could maybe make an argument for Civ4 based on consensus among the "hardcore" strategy/4X community. (Obviously those of us in this particular sub-forum may have a different opinion, but there are only dozens of us

!) As I see it, Civ4 nailed down a lot of the expanded gameplay elements introduced in Civ3, making it more complex than the earlier games without the disruptive changes of Civ5. Some people go so far as to say Civ5 isn't even a 4X since expansion is penalized at some stages of the game, it's not really the same games, the series ended with Civ4, you kids get off my lawn...
why are (at least) TWO fanmade opensource civ games are heavily based on Civ2 (even down to unit lists. while not neccessary a copycats, but ALMOST every units are verbatim copies, including capabilities! (Trireme, Caravek and Galleon are 'Combat and Transport' ships) ).. and this was in case of FreeCiv. while CivEvo systems is what baffles me more since AFAIK almost every units must be manually designed. (And unit nomination algorithms will give out names according to techs, sizes capabilities and so on,)
As for why the clones are of CIv2... it was the easiest/best one to clone at the time the clones were first started.
You get UnCiv which kinda tries to clone Civ5 but kinda leans towards CIv2, because it was simply easier to do with limited resources.
Agree with Rambo here, FreeCiv and C-Evo are very old projects, 1996 and 1999 respectively. It wasn't that the creators had the entire series to choose from and they chose Civ2, but rather Civ2
was the latest and greatest at the time. Unciv has some aspects of Civ5, but heavily simplified both in gameplay and aesthetics.
Why haven't we seen clones of Civ3+? These games started adding progressively more gameplay elements (strategic resources, civics, great people, religion, culture points...) and 3D graphics, so it's a much higher barrier-of-entry for a solo dev or small team.