Well, as for their animated movies, my personal favorite is Mulan, but as for overall quality, it's difficult to beat either Disney's earliest stuff or Disney in the '90s. I would call those two eras Disney's two "Golden Ages": 1937-1959 (Snow White to Sleeping Beauty) and 1989-1999 (Little Mermaid to Tarzan). There were also some gems in that between period, and most of the entertaining ones to me are from that time (101 Dalmatians, Sword in the Stone, Robin Hood), but there wasn't the consistency in quality in that time that there was before or after it.
As for the greatest of the greats, taking overall quality of animation, music, story, and production, I would say from the early period, they would be Snow White, Pinocchio, Bambi (how many of you were introduced to the concept of death through Bambi's mom?), and Cinderella, and from the later period it's gotta be Aladdin and The Lion King.
As for live-action, hands down it's Mary Poppins, no contest, though the first Pirates of the Caribbean gives it a run for its money in terms of entertainment value.