Hmm... I like types that contradict each other in a way, like Volcanion's Fire/Water. I like Ludicolo's Water/Grass as well, though that's just a tad less epic. A Fire/Grass Pokémon would be epic as well for weed jokes. I think the most exciting type for me would Dragon/Fairy. I don't believe any Pokémon has it currently, but man that would be cool.
I judge these by how cool the words sound together, not anything resembling competitive knowledge :P It could be terrible in battle, I'll never know.