King Solomon's Mines seems like the best natural wonder, now. The only thing that comes close is the GBR, which is actually two tiles worth of bonus yield. Why I feel this way:
1) +6
is incredibly powerful, even beyond the early game. Prior to planting a GE or getting the late-game SP for extra production from mines, it's highly unlikely that you will match this sort of production in a single tile until rather late (or at the expense of a precious GE).
2) In an early-game situation, you're basically looking at building a granary and/or water mill in a ridiculously short time to start getting food yields up quickly. This means this city will catch up very quickly to whatever your capital is doing, assuming similar terrain types around each city.
3) As Spain, this is +12 production (and whatever gold for finding it).
4) Again in the early game, any side with an early UU that finds this is likely to bowl-over its neighbor. This goes double for Germany, Zulu, and anyone else who can more easily afford the high maintenance of such a large army as well as having said early-game UU.
5) On higher difficulties, an AI civ that works KSM may have unbelievable production, given that they already get bonuses to help them.
Ultimately, KSM seems like the best NW in the game now. The only competitors are Eldorado (if you find it first, at any rate) and GBR. While Eldorado can let you buy a quick Settler, KSM could let you build the Settler in almost no time and let you do it again and again. And while GBR is fairly potent, GBR requires you to be on a coast and to also be able to actually reach both tiles.
It's up for debate, but KSM seems like a new 'Skill-dorado'...
1) +6

2) In an early-game situation, you're basically looking at building a granary and/or water mill in a ridiculously short time to start getting food yields up quickly. This means this city will catch up very quickly to whatever your capital is doing, assuming similar terrain types around each city.
3) As Spain, this is +12 production (and whatever gold for finding it).
4) Again in the early game, any side with an early UU that finds this is likely to bowl-over its neighbor. This goes double for Germany, Zulu, and anyone else who can more easily afford the high maintenance of such a large army as well as having said early-game UU.
5) On higher difficulties, an AI civ that works KSM may have unbelievable production, given that they already get bonuses to help them.
Ultimately, KSM seems like the best NW in the game now. The only competitors are Eldorado (if you find it first, at any rate) and GBR. While Eldorado can let you buy a quick Settler, KSM could let you build the Settler in almost no time and let you do it again and again. And while GBR is fairly potent, GBR requires you to be on a coast and to also be able to actually reach both tiles.
It's up for debate, but KSM seems like a new 'Skill-dorado'...