It's subjective. I like h.264 Quicktime the best, although there is a bug with gamma correction in Windows that Apple never ever fixes, so in Windows colors look more washed out.
I think MPEG-1 is still the de-facto, everyone-will-be-able-to-watch-it-no-matter-what option, but it comes at low quality.
Others say WMV, but I strongly disagree.