I think Porygon-Z in underratted he is always in the UU category and yet when i used him online it was pwning everything cause he can learn a variety of great moves.... but over all i think that poliwrath has a great type combo but is beat out by starmie ....
Golem. I used one in Platinum and it was totally boss. It's only downside is it's many weaknesses. But it's a great damage dealing tank. Also, Gigalith. It's like Golem with less weaknesses, so I reallly don't het why it's underrated, but it is .
Gyarados. Lots of people just look at that 4x weakness to electric and think, "That thing is crap, why would you use it?". I don't look at its weakness as a problem, in fact as long as you have a ground type those electrics will be useless. Honestly Gyarados has a decent movepool, good stats in almost everything, and it evolves fairly early depending on when you caught your magikarp, which for me is usually around lvl. 10-15.
I would like to say Magikarp is the most underrated Pokémon in existence. People only like Magikarp when it evolves into a Gyarados. I love Magikarp because it is simple and it will pay off in the end if you train it correctly through EV training.
I'm also going with Eevee. He may not be as strong as others, but the fact that he can turn into several different types is just awesome. Plus I've found good use for Flareon and Glaceon on my teams. So really, it's the untapped potential that draws me to Eevee.