disagree with quite a bit in this thread-
Rome and Greece - both civs have 2 unique units that are available during the same era, and all four UUs do not have advantages that upgrade... Well, I believe the ComCav's GG bonus upgrades, but it's real advantage is the 5 movement points. With either of these civs, you'd better gain a substantial advantage before chivalry, otherwise you're playing a vanilla civ with just their UA's (although both UAs are above average.)
France - I'm torn on this one. I do currently have my second best score using France (1670 AD space race), and actual gameplay shows them to be very good. But on paper, a side-by-side comparison with Songhai shows that they're just not as good as the Songhai, and I wouldn't say that Askia is necessarily the best civ:
France: +2 culture/city
Songhai: +2culture and +2 gold (in saved maintenance)/city. The unique building gives +2 culture. so as long as you build it in every city (and why wouldn't you, it's maintenance free), you get the same bonus as the French. The French advantage is earlier access, and the Songhai advantage is +2 gold in saved maintenance, so the Songhai UB basically cancels out the French UA
I'd take MandyCavs over Musketeers (or Foreign Legions)
So what's left? Foreign Legions, a UU that comes after the game has pretty much been decided, or a.) extra gold from encampments, catapulting your start, b.) extra gold from cities, leading to major developments after your first (and subsequent) conquests c.) land units that can survive when embarked, and d.) the ability to ignore rivers on attack but still gain their bonus on defense.
China and Persia... OK, agree. They're both grade A civs. But there's one civ that outshines both, IMO. Before I say who, let me say why- Regardless of what victory condition you aim for, gaining technology and gaining social policies are two of the strongest paints on your palette. -technology is gained from beakers, and beakers are based on population. Faster pop growth leads to faster acquisition of technology, and there's only one civ that has a population growth bonus... Which happens to be the same civ that has one of the best culture bonuses... the Aztecs. And to further push them ahead, the UU has 3 bonus attributes all of which upgrade, so upgraded jaguars make for better swords, muskets, infantry