Cloyster i think

Porygon-Z It only evoles by trading with a certain item with the entire evolionary line and it looks just strange but It was a very high specail attack and I like it.

I say wailord is the most underrated people barely use him but he's got incredible hp

austipokedude : The fourth highest in all of pokemon.

This one might not be so good in specialized rule battles, but a perfectly underrated Pokémon in my opinion would be Smeargle.

He can learn any move, which means he can make a deadly combo of skills that becomes downright unfair. Spore, Mindreader, sheer cold (or other instant death skills), and maybe mean look or something and he will kill your Pokémon! He just needs to survive... I think he would be ideal in a team match, doubles or singles! Get a tanky Pokémon with "Follow Me" or whatever move that would take damage away from Smeargle, and let Smeargle do his thing!

In the past, my ideal partner for Smeargle was Togekiss as it had some decent tanky stats, learned follow me, and could self heal, but by now there could be a better choice.

Note, I have not played much of Pokémon Black & White, so if some moves have changed or there are better options, forgive my ignorance on the subject. I know Instant death moves fail on targets lower level than the user, but believe they do work on SAME level targets.

Lol at the people saying magicarp... if he's so underrated.. Why does almost all trainers go and catch one? (unless they do the easy way of catching the evolved Gyarados, which is actually non-beneficial.)

Smeargle, with its ability to copy any move can make it deadly cause you can never know what its going to do.