Many find that WMV HD's work well on YouTube, and some others like H.264, though a few have problems with that format/CODEC. There are dozens of suggestions on this forum, from people, who upload to YouTube with good results.
I'd do a Search on these fora (PrPro and PrE) for "YouTube," and read the suggestions. I think that Ann Bens has contributed to many of those.
If you have CS4, choose H.264 in AME then you will have Youtube format options.