Everyone and their mothers will probably tell you to use Gyarados so I'll avoid that (except for just now). Just looking at my own boxes to remind myself what exists so far and what doesn't, I see an Azurill-- Azumarill is actually pretty strong if you can get Huge Power on it, and it'll have Aqua Tail (90 base power) by level 21; it actually learns Rain Dance on top of that, at 35; and if you REALLY want to go full-competitive, you can get Superpower at level 42. I imagine Escavalier to be pretty good in Rain as well, as its only weakness is eliminated. I'd also vote for Lanturn because it has much-needed bulk and potential Water/Volt Absorb, although at the same time it's, and I hate to say this because I love it, pretty bad overall. I stopped using it after I beat Aya, but then again I stopped using a lot of things then (or before) like Noctowl, Gigalith and Crawdaunt. ... Speaking of Crawdaunt, it's not terrible either, especially with Adaptability, and you might even be able to get Dragon Dance on it with a little effort c: (not yet, of course, since it comes from the Horsea and Dratini lines)