Dan Dailey & Donald O'Connor. Both of them showcased when dancing with Gene, but Dailey especially had a lot of great dancing in movies of his own.
When My Baby Smiles at Me! Dan Dailey & Betty Grable are particular favorites of mine, but this one just stands out for me. I love so many of the ones already mentioned in this thread as well, but I think that this being lesser known (or at least less remembered today) makes it feel a little like my own little secret favorite. I find that my reasons for something becoming a favorite are only partially about the movie--also important are who is watching with me, and my frame of mind at the time.