I think I'll vote for Griffey Jr. fo the sweetest swing, but my next pick would have to be Alex Rodriguez.
A-Rod does have a nice swing. His only problem is that he makes himself look stupid reaching for a breaking ball nearly every at bat. From what I've seen, he usually only does this with less than two strikes, but its still pretty embrassing looking.
As for an ugly swing, how about Sheffield? If you see a pitch he fouls back, he falls across the plate and backwards. Its just ugly. Plus those violent wags don't help. Whatever works though I guess
Griffey Jr. is probably the nicest swing I've seen