Well I was rather bored so I skimmed through the B/W Dex and started looking for things that I thought might be underrated or forgotten. If I was only supposed to pick one, then don't open the hide tags so I don't bias you. I may be biased on some because they might be "cooler" to me just because of my favorite-Pomeon-list and such. XD
Warning! Some are harder to use in OU than others...
-Sharpedo - Maybe not as underrated as it could be, but still not very seen and can hurt quite badly, as seen on Agamemmnon's (spelling...?) RMT somewhat recently.
-Shaymin - This poor little hedgehog is so cool, but Ferrothorn-o-ration has made everyone forget this little guy. Shaymin can co-op with other things on your team (hi Volcarona! Lemme help you with that Heatran!), absorb Electric and Water moves, and NOT die from Pursuit/weaker U-turns. Major cool points.
-Zoroark- Is it hard to use? Yes. Does Team Preview hurt its usability? Yup. Can it really mess with your opponent if played right? You betcha. There's the appeal.
-Nidoking - Question not the might of the 4-foot tall king. Another not-expected threat in the OU environment with its excellent coverage and awesome utility with immunity to Electric and Toxic, and absorbing Toxic Spikes for your walls/pivots/grounded sweepers. Not to mention it, like Shaymin, is just really cool.
-Aggron - Last generation when it got Head Smash to combine with Rock Head, Aggron was major "cool" and "commendable" points to those who could use him well enough. The Fighting abundance now won't help, but if we can unleash that deadly Head Smash it'd be a sight to see.
-Archeops - Why not let this bird live up to the hype we gave it before we knew what its ability was? All I have to say here.
-Sableye - Yeah, it's on a hype spike right now, I'm aware. And rather guilty of cranking up its usage numbers myself; I think half its usage this month is from me... But why not make people not just say "Sableye sucks" by default? Personal favoritism for my lil ghosty...
-Eelektross - Poooor sad forgotten Electric eel... But with Thundurus gone, he has a slightly bigger niche to fill on a team as an Electric STAB.
-Cloyster - I'm aware this is still on OU, but barely. And why? It can murder pretty much everything after a Shell Smash, including the great wall Ferrothorn. If I can only put in one vote, this is it. You know that feeling of fear you get every now and then deep in your gut and very soul? That's what you get when you let one of these set up in front of you and it proceeds to go to town with your team, often dying from its own Life Orb recoil because you can't touch it. And even if you can, will it do enough? Even at -1, its base 180 Defense is insane and defends from all non-Vacuum Wave priority moves, so it'll eat that Mach Punch and kill your face. It does require a good bit of support, but with the metagame being what it is with Prankster Thunder Wave Thundurus and hyper-fast Excadrill gone, as well as Dragonite and Hyper Offensive teams one the rise, both of which aren't able to well defend from Cloyster, this may be our clam's time to truly shine and destroy the world.
Those are my thoughts. If I can only nominate one, see the last one on the list.