Celts. I'm tired of having to wait for expansions.
1 - Their presence in Europe predated virutally all others (almost certainly all others that made it in the game), their area of influence at its greatest extent was as large or larger than the Roman Empire
2 - Some customs and language features remain to influence the later cultures that absorbed or displaced them (in some cases this is a simple as place name origins, but in many it runs deeper).
3 - During the dark ages, it was Celts that reconverted the continent of Europe to Christianity and preserved (alone, to a large extent) the knowledge of Western Civilization.
4 - they do have a popular, modern, independent country (Ireland) and another popular country part of the UK (Scotland. Wales, too, but I think Scotland gets more press.)
5 - the only thing they lack is a unified empire, but since culture plays such an important role now I feel that requirement is less important. Oddly enough, it could be argued using this logic that they really didn't belong in vanilla Civ II (where they were) but instead should have been given space in vanilla III and IV (where they weren't).