I've never been a fan of Mewtwo myself. My older brothers and I watched Dragon Ball Z religiously when I was little and he reminded me of Frieza which just sort of kills the cool-factor...When I was little I loooooved Mew though and I was absurdly excited when I managed to catch Articuno after accidentally knocking out the other two birds, so they both have massive nostalgia appeal. I think in general the Legendaries tend to be over-designed and end up visually cluttered or trying-way-to-hard-to-be-cute. I do like Darkrai and the Musketeer deer though, and Shaymin when it's in its original hedgehog-y form.