I think the consensus is coming around now that
1.) The Celts were oversold a bit surrounding the G+K release, but now that the initial disappointment is past, and the power of religious synergies is better worked out, and the social policy trees changed with BNW, The Celts have been finding their way into a pretty high tier. Not God Tier, but definitely solid.
2.) Sweden are an extremely odd duck, arguably much more of a "hypercube" than Venice, because at least with Venice the way to play them is extremely clear, whereas with Sweden it took a lot of working out, but Sweden played properly are apparently also very, very good.
I'd consider Versatility one of the top factors in choosing which civs are "best" and "worst." If you're playing a high level, and have picked, say, Babylon for science victory at the outset, and then meet Korea, you've got some thinking to do (a little less so the other way around, as Sejong always beelines Writing and cares about the Great Library, while Nebby bizarrely is always last to get to Writing among the AI, for some reason.) Likewise picking Diplo early on and meeting Alexander, picking Culture and meeting Pedro, picking Domination and meeting Genghis Khan, etc. Being able to adapt and change strategy is absolutely crucial.
In that light, the general tiers don't change all that much, necessarily. Poland is still at the top, the most versatile of civs by far, easily able to pursue any victory condition and shift easily to any other. Korea is probably next, if only because its UUs edge out Babylon's in the long run (Mongolia is going to have a much tougher time rolling a science-focused peaceful Korea than a similar Babylon, whose defensive bonuses have more or less obsoleted by the Medieval period) but both are going to be able to use their science lead to allow for versatility if need be.
Then we get a civ like Inca, with various small but extremely useful bonuses to production, growth, gold and movement. Science is obviously the most important single aspect for victory - the best players, and I am certainly not among them, have said so repeatedly - but I know that for me, hammers are what I care most deeply about in the beginning. The Inca bring them, and a lot of other important early bonuses. Rome and Egypt also bring them, with solid UU/UB to help along the way. Aztec is hampered by both having no production bonus and a jungle start bias, but their Uniques are synergistic in a way that allows for an absolute powerhouse of major population growth, the strongest culture start in the game, and early CS alliances that more than make up for it. Far from being a simple Domination-engine like it might first appear, the Aztec are among those most versatile in the game. The Maya are like a different design of what makes Poland so great, and have to be mentioned here as well.
Then we get the Economic Civs. A runaway economy can create the other factors needed for just about any win condition. Venice is obviously the biggest gun, economy-wise, but is a glass-cannon and can't really be considered top-tier in all situations, especially in MP where they will just be rolled ASAP. When we talk about Economic Civs, we're really talking about Arabia, Portugal and Morocco, any of whom you should be frightened of in the hands of a player who has any idea of what they're doing. The Dutch UA deserves a mention here as well, though they were designed for G=K and left in the dust by the changes in BNW in a lot of ways, particularly the lux-for-lump-sum exploit that made them so crazy good in the G+K days. Their UU and UI are still pretty great, though.
There is a fine but crucial difference, though, between "Versatile" and "Unfocused," and that's where we find the bottom tier. Well, lack of focus or else a focus that doesn't amount to anything. A lot of this depends on the player and strategy, of course. Post-patch France is weak, to be sure, and the Musketeer adds little if anything to the UA and UI, which don't really shine until the second half of a culture game anyway, but France still has advantages that can work for it. Similarly, Indonesia is tough to crack, but their UA and UB scream for advantages with the Liberty tree, and their UU (one of the best in the game) sort of demands expansion with its Iron requirement.
"Unfocused," here, means something like Byzantium, whose UUs don't necessarily mesh together, and come at the same time you'd need to be focusing on faith-output (which the UUs don't do anything to help with) in order to get anything from the UA at all. "Don't add up to much" means Denmark, whose abilities need extremely specific conditions to be helpful, and even then might not be the best on the map for those conditions.
At the center of the Venn Diagram of "Unfocused" and "Don't Add Up to Much" are the Iroquois, focused not on any Victory Condition, but on wood, with benefits it's tough to care about in any real way. They can be played successfully - any civ can - but it is my belief that they are thoroughly the worst, with no clear path to any one victory condition, and no clear way to get ahead of any other civ in anything in particular, and no good tactic for switching paths.
