.mpeg is usually the clearest unless the .avi file is divx than that would be smaller in file size and best quality because of the compression. So look for divx or xvid avi
All DVD rips start off as mpegs. So mpeg will always be the best quality. It's just going to be like 2-3 times the size, but unless you're going out of your way, DivX/XviD looks great too.