For me- Hydreigon! OMG Finally a Dark/Dragon type Pokemon. When I first found out about Hydreigon, it became my absolute mission to get one on my team for Black. Hydreigon has become like my go-to pokemon for nearly everything: Battling the Elite Four, Battling on the Battle Subway, stomping Ghetsis and his hacked Hydreigon into the ground... My Hydreigon Nyx even managed to knock out Zekrom with just a hit...
Yeah, I absolutely love dark and dragon types, and gen 5 finally brought me the perfect combination of those types in one amazing Pokemon.
The only thing I was upset about: Deino could only be found towards the end in Victory Road. I really wanted to have my Hydreigon to totally own the Opelucid Gym.
But owning Ghetsis' Hydreigon was good enough for me :D