It's not as easy an answer is one might think at first glance, but I'm going with X100.
The older M4/3 chips are decent to ISO400 and usable at 800. The GF2 is okay, kind of dumbed down from the GF1. The AF works well. The 20mm is a very nice lens. If I had to choose between my GF1/20mm combo and the X100, it would be a tougher choice.
However, the X100 has a larger sensor, much better high ISO quality, handles well, and the AF issues seem to have been mostly overcome with firmware updates. It has a built-in viewfinder, which is much easier to deal with than the clip-on EVF of the GF2. The lens on the X100 is also excellent. The only widely reported downsides all have to do with the user interface, not the image quality. If you've handled it, and like it, that's the one I'd choose.