i'm in your conscience
I'll go with Mew! It's small, pink, and adorable. And it learns every TM and such and that's awesome to me, considering I love Pokemon with crazy movesets. Fly is fun for a Mew. Mewtwo is awesome too, but Mew had so much mystery around it back in the RBY days, especially since not many of us knew the cheat to get it in the game without attending an event.