It's impossible to count, to be honest; there's a lot. Off top of my head, one would have to be Jirachi. It's got a pretty cute-looking appearance, which I believe suits this Pokémon well, and when it comes to battling, it could perform astonishingly. Jirachi has Serene Grace ability, which raises chance of move-effect occurrences to a decent amount of level, allowing it to take advantage of attacks like Iron Head. With Iron Head, this Pokémon is able to make opponent flinch almost each and every turn. If it manages to paralyze foe alongside, chances of them striking would dwindle even further.
Another of my favorites is Shaymin (its normal form, and not sky). Its design is quite cute, in my opinion, and could carry itself well when it comes to matches, as it has decent overall stats and move-pool. Due to similar reasons, am also fond of Celebi.
Few others would be Giratina, Zekrom, Cresselia, Deoxys, Darkrai, and Lugia.