At the moment, I guess Sony and Fuji are ahead. But in 3-5 years I think both Canon and Nikon will have very strong systems. And I really hope that Panasonic and especially Olympus can keep up with sensor, for they have wonderful lenses and even cameras. And I love compact cameras, as I’ve said about 55 times.
I’ll say it again though: I think that for most purposes, full frame is overkill. Too big and heavy gear, too narrow depth of field, and bigger sensor than we need even now, and particularly in the future. I think the rush to full frame is ten years too late, it’s just a fad based on perfectionism and fetishism.