Gyarados vs Milotic?
Definitely Gyarados here. I have never used a Milotic myself, since I've never had the patience to actually catch a Feebas, and the only times I've really battled against it was against in-game trainers that had it, which weren't that many. I'd say that Milotic would win for design, because as Ralaia said, Gyarados is ugly. Milotic did also pose as a bit of a challenge against Cynthia, but alas, if you don't have a Electric type Pokemon, Gyarados is also extremely hard to beat.
Gyarados was also at the top back in R/B/Y's prime days, and was up with the likes of Charizard and Mewtwo in popularity, and I felt that way too. It was a hell to train a Magikarp to get it, but once you finally had it, it was a monster on your team, and I remember it was quite a big challenge in the E4. But yeah, Gyarados is definitely the winner, since it's one I actually know, and it does appear to better in some categories.