There's certainly the most documentation on x86, but I've already discussed my philosophical objections to the ISA in other threads. That said, you can get plenty of help and there's lots of people who know it. So, from a support standpoint at least, I guess Intel would be easiest.

In terms of the actual instruction set, 68K is actually very elegant. The instruction set is highly orthogonal and works "like you'd expect." I think as CISC implementations go, it is probably one of my favourites. Apple provided lots of support in the Toolbox for people writing raw assembly code, so it's not as bad as you might think. You just have to be aware of what your registers are doing and which ones you need to preserve. However, segmentation can really ruin your day -- CFM-68K helped a lot here.

Register usage is much more important for the PowerPC because of its load/store nature, its much larger register file and the PowerPC ABI - the list of registers and conditions you need to preserve is much longer, and the instruction set is dizzyingly large despite being "RISC." It's really RISC only insofar as it is load/store. That said, it didn't take me long to learn it and many of the instructions are just variants of more basic operations.

I'd really like to be better at ARM, personally. "They say" it's inspired by the 6502 but I don't really see much resemblance.