Hmmmm... Gotta go with Swampert. Confused? Well;
Water attacks beat Moltres, Groudon (although Ice is better as Groudon does sunny day), Entei, Regirock,
Ground works wonders on Raikou, Regirock and Registeel and the previously mentioned fire types, as well as being resistant to Zapdos' most powerful moves.
TM Ice Beam takes out Groudon, Rayquaza, Shaymin, and
Bite (which it learn via breeding - see bulbapedia) can take out every psychic type under the sun.
Celebi is the only sort of counter though, but it has poor defense and bite could literally take a chunk out the poor little elf thing.