Once again? ... Okay.
First of all, alot of that depends on the difficulty. Quechas are great on pretty much every difficulty where you'll see barbtrouble and the AI starts with Archers. Skirmishers are close to useless on every difficulty below Immortal, maybe even on everything below Deity, but on Deity itself they're really good sometimes - even better @ MP. Holkans for example are pretty much a non-issue for everything below Immortal, but they're terrific on wide-open maps where you do NOT have strategic resources to defend yourself and barbs will enter your territory @ turn 40 on Deity.
You can win the game with Vultures, Praets, Immortals or Warchariots alone on every difficulty up to Immortal, but on Deity they're just a better version of their original unittype most of the time - doesn't make them bad, but they lose alot of their value.
Samurais, generally considered to be a rather weak UU (because they're medieval), will just tear apart everything in their way up to Emperor; they're a very good unit on those difficulties, and a fun UU on top.
As AbsoluteZero has shown in one of his videos, Numidian Cavalery is a pretty good / great UU on Deity, but it's worse compared to standard HAs on pretty much every other difficulty.
And then there are UUs who are good on
every difficulty. Namely Keshiks and Fast Workers. And only one of those UUs is resourceless and useful on
every setting... Fast worker is the best UU if you take all settings/difficulties/gamespeeds into account, not the slightest doubt.
So, the difficulty matters alot here. But so do the settings - Deity/Fractal with isolation you'll love Holkans, trust me on that...
I'm not even sure about how i'd rank the UUs on, say, Deity/Pangea. Sure, #1 is the fast worker, but #2, #3 etc. aren't that clear... but maybe like that:
1. Fast worker/Quecha
2. Keshik/Numidian Cavalery/Conquistador
3. War Chariot/Skirmisher
4. Immortal/Praetorian/Bowman
5. Holkan/Dog Soldier
Ye ye, gotta admit that those NCs are really somewhat awesome in this case...
Skirms+Bowman because they will make you survive early battles.
WCs+Immortals because although you most likely won't rush someone with them, they're great vs. Barbs and to take barb cities, which is worth alot when you're desperate for more cities.
Quechas for the well-known fact that they're imbalanced.
Keshiks because they melt faces and are the most sexy UU in the game. Conquistadors because they're strong as already pointed out several times.
And Holkans/Dog Soldiers because they're resourceless. Priceless if barbs are about to jump on your throat.
Fast workers because they're the best UU in the game, although Quechas are tied on this setting thanks to choking abuse.