I would say torous, the bull pokemon. He is a tank at high levels.

Venemoth, Tauros, Yanmega, Xatu. These are some more underused Pokemon in my opinion.

What, no one mentions Porygon-Z? I've never seen someone else use it, and it is capable of a OHKO on Blissey with Hyper Beam after only one turn of boosting. That's right, a non-legendary can OHKO a Blissey with a Special Move with just one boost... And I'm counting the Blissey as being EV raised for HP too.

It is also capable of a 2HKO on any Pokemon in the game after one boost, depending on it's moveset.

It's not truly fast enough to be a sweeper, and the fact that it does need to use Nasty Plot once means you can't give it Choice items, but it's still insanely powerful. It IS fast enough to serve as a Tank destroyer though--it's relatively fast even though it doesn't have elite speed.

It's Defenses are low, but not terrible so It should be able to take a hit or two that isn't Super Effective.

STAB Tri-Attack is quite useful.

I think Shuckle is pretty underused as a Pokemon, but it has great capabilities with the right combination. It has the highest defense out of every Pokemon, plus it can learn Trick Room, which switches its attack and defense. So you can use this potentially deadly combination to OHKO just about anything. If you use it in a double battle with a Pokemon that knows Follow Me, then it won't get attacked and won't be in any danger of fainting.

Magikarp definitely it can use surf and waterfall to get to the E4, then when you get there you dump it back in your PC. :p

yoshpit222 : I agree completely, which is why I chose Butterfree.

Mine is definitely Butterfree. Butterfree is a versatile, strong and unappreciated Pokemon.
I've owned several trainers throughout the Pokemon games with Butterfree alone. I think it's because of it's half Psychic type that makes it so strong because Bug types are typically weak.

Butterfree needs more recognition for it's awesomeness!

i would have to say togipi because it is in a egg but if you give it love and level it up and evolve then it is a great pokemon

Caterpie is unnaturally underrated. Any players of Brawl Plus would know that Groudon evolves into Caterpie. Besides that, Caterpie becomes Butterfree and Butterfree actually has a decent moveset.

Feebas is underrated and I'm suprised no one mentioned it.  When it becomes Miloticit becomes one of the best water types in the game but is normally overlooked

Magikarp may be useless at first, but like Leggy said above, he can go from useless to an awesome pokemon dragon that can dominate! Yeah.

I think Ekans and Arbok are quite under-rated Pokémon Poison type Pokémon are usually quite good their move sets are fairly good and there stats are also quite good also they both look awesome!

I would have to say Cradily, when combined with a sandstorm pokemon, as its rock its sp.def doubles (or maybe its x1.5 im not sure now) in a sandstorm and curse and recover it becomes unstoppable, especially with seed bomb and rock slide.... it just one hits everything and it takes a lot to kill it

Or Cloyster, its got an ability that makes icicle spear and rock blast always hit 5 times, its knows toxic spike, spikes, stealth rock and rapid spin and then theres its huge defense and i see nobody using them... ive seen cloyster maybe once but never cradily....

A truly underrated pokemon would be dunsparce, because with glare and serene grace you got an amazing paraflincher. It's not has good has togekiss in that regard but one is in UU while the other is in OU so there not competing for a spot on the same team anyway.

The only Pokémon that make me mad is the ones that do not listen like for example if you evolve charmander he will never listen to you only in the mewtwo movie also magicarp because he pretty much useless until level 15 but he can get more exp more than usual.

I would say cloyster, I have never seen someone use that thing, except for a non playable character. 

I think Ampharos is underated.  To me it is one of the most powerful electric types.  I feel like I'm the only one who uses it.