What makes the Mayans superior to Spain for 20k?
Probably nothing...if that's your goal going in, Mayans are probably mid-tier.
Or to any Civ for Diplo? To Byzantium for Space?
Larger cities, better developed land, more free labor, larger land from the grab, more specialists supported, etc. Easily first-tier in this regard. Absolute best in a narrow sense? Quite possibly, but not definitely. One of the best of being in a position to win in those ways? Without a doubt.
No, there is no civ that is the BEST at all things. I'm really glad of that. Heck, I'm even willing to say that no civ is better than any other civ across the board -- in certain circumstances, one could probably make a case for ANY civ being the best (some are definitely harder than others, but if you play a 5CCC variant disallowing ground combat on a pangea, one can make a reasonable argument for America as the best civ). All Civs have strengths and weaknesses.
My general thought is given a random map on deity or Sid (or beyond/between), what civ will be most likely to give me a good chance for a win -- any win? That's my take on tiers and it's dramatically different from yours. For most people, I think that's a fairly accurate representation of the game goal -- get a win, one way or another.
Sure, if you're talking fastest domination or earliest conquest or quickest launch or ...., then priorities may well be different. Any time you add variant thoughts, things change and relative values of certain things change.
As for Ision, I disagreed with him on a lot of things. I didn't see his Sumerian review so I can't comment on it, but you can see I posted disagreeing thoughts on his UU list. But, heck, I don't claim to know EVERYTHING about Civ and strategies. There's always a place for learning new things. The problem I have is that I see your list as teaching things that are inaccurate enough to really lead people astray. And that I don't like.
For all the talk of how the agricultural proponents aren't giving details, there are almost no details about why traits X and Y are synergistic or why X and Z aren't. There's no talk of why this value should equate to that value. Just a bunch of random numbers with very little reasoning. Does nobody any good, IMHO.
The little things at the end are the worst -- starts with Alphabet? So? That's just giving a bonus to certain traits (Commercial and Seafaring), but has little bearing on how a typical game will play out. WAY too much consideration here. Best choices for 100K and 20K but no mention of space, diplo, domination, or conquest? Extreme bias alert. Sets off the scanners, even if there is a reason. I completely disagree with the 100K list, too, as the easiest way, by far, to get 100K is with a bunch of cities and Temple of Artemis....once again, growth wins. But that's almost beside the point, because the lists aren't balanced to begin with. And there's no explanation of WHY you feel certain civs are best at this, just a list. Plus, when that list disagrees with an expert like T-hawk's list (20K), I know who I will tend to believe. And "MP" -- what role does that have here? Especially without a HUGE discussion of agreed-upon legit behaviors? BAH!
Arathorn