An interesting question. It's hard to know the relative value of bonuses at this stage. I'd guess Science is a bit less important than Civ V (because half the techs are spun off into the Culture tree), while Culture and Faith are more important (because of the Culture tree and Religious Victory). I'd guess Science is probably still #1 though.
I'd say Brazil. The bonuses look really crazy. It's kind of a boom/bust Civ in the mold of Civ V Spain, because it's so dependent on the right terrain. I'd assume, though, that with start biases Brazil will start in the jungle more often than not.
China also looks strong, because Eurekas don't look very hard to get at all. That's a big discount on an awful lot of techs. I'm a little dubious about the Great Wall, but the builder and wonder part look good.
A third strong civ is Scythia--they remind me of Civ V Huns. I imagine Horse Archers will be equally devastating this time around. The Scythians have an advantage over the Huns, however, because it looks like Civ VI will not penalize early aggression nearly so much.
As for Civs that look particularly weak, America is an easy choice. Really late uniques, as always, so those probably won't help too much in many games. I'm not sure Government Legacy bonuses are important enough to compensate for that. Britain's UA also strikes me as potentially quite weak, but we need to wait and hear more about the archaeological system before we know for sure.