Weavile, hands down! I was excited to hear my favourite Pokémon would be getting an evolution in the forth generation and I was hyped. Then, it finally came along and it did not disappoint. Great design, great movepool, got a huge benefit from the physical/special split and a great Pokémon to use.
Even to this very day, Weavile and the now pre-evolution Sneasel are my favourite Pokémon of all time. I think there's just so much to like about it.