No Absol? D:
Good ol' Darkrai. It's name certainly isn't very inventive and nor is it's design, but at the very least it's still executed well. Since the design is good and it's a Dark type (/bias) with stereotypical dark pokedex lore; as well as one'a the few truly evil Pogeymanz a'la PMD2; I definitely like it.
Darkrai would be the scariest thing ever with a BST total of 680. You need t' prepare for this bastard in Ubers, simply put. Whether you've got a Scarfed Poke or a Sleep Absorber in the form of somethin' with Natural Cure (watch out for Focus Punch, Blissey ) or a not-Giratina RestTalker, ya simply need it. Otherwise Dark Void will instantly shut down one'a your Pokes and Darkrai has already taken the lead with one bloody move.
On top of that, whether you've a soft or hard counter to it, literally nothin' can switch in on safely it if it's running a not-so-typical gimmicky moveset Life Orb/Specs moveset (of which Mewtwo is outright better at, minus Dark Void) with things like Spacial Rend to maul a Palkia with. When not usin' any attacking moves besides ye olde Dark Pulse/Focus Blast, if the foe sends in a sleep absorber, Dark Void can simply be saved to shut down Darkrai's specific counter (if any) instead if it's running Nasty Plot + two attacks. If the Sleep absorber ain't somethin' like a specially defensive Heracross, it ain't gonna be able t' eat many boosted Dark Pulses and will eventually faint, leavin' Darkrai to put t' sleep somethin' more important on their team. Another threat is simply a Scarfed Darkrai which can outspeed most anything and hit anythin' twice unexpectedly. It's also got trick to screw walls over with, allowing it t' get a free Nasty Plot on the switch; or maybe even get up a second one if it's a stupid Blissey and proceed to sweep if the foe lacks somethin' faster/with priority.
The only common Pokes in Ubers that outspeed it without Scarf are Deoxys (who might not even be common; never was when I was an Ubers player), Mewtwo and Shaymin-S. Obviously neither Psychic type can switch in. Though it ain't common, Ice Beam can be run to obliterate a Skymin brave stupid enough t' try and come in on Darkrai's attack; Dark Pulse does enough damage to Skymin's lacking defenses anywho. Opposing Darkrai need to beware a Focus Blast.
In other words, if ya don't have a Poke that's specifically on your team t' counter it, it's gonna screw ya over somehow. All you can do is scare it off until the trainer makes a stupid mistake or decides to sacrifice it. 'Till then, njoy ur nightmarz.