I spend most my gaming time on my PC. Always have, always will. It just happens to be the system you can do the most with, so why not take advantage of that fact?
Second would probably be my PS3. Yes, the graphics are better, though I definitely wouldn't use that as a selling point. But I also prefer the interface and the contoller, though I feel I might be the only one. Plus I've never once had a single problem with it, and I love BluRay!
Third would be the XBox 360. This is actually my preferred split-screen multi-player system. I probably play more games on this than my PS3, but only because I play more multi-player than single-player gaming. System has had to be replaced twice now though, but one of those times Microsoft paid for, so was just a lack of system for a few weeks issue.
Last would be Wii. I pretty much hate it personally. The worst is the contoller. After about ten minutes, my hand always starts to hurt and feels like it's going to cramp up. But I do admit it would almost have to be the system of choice with children and non-gamer parties. Plus Mario Kart is always fun! Has the worst instruments for Guitar Hero and Rock Band too. But there's always the balance board, which I've yet to try out. Don't really ever plan to either.