I apologise if this set has been posted elsewhere, but I've been using it to relative success recently, so I thought I'd share it here.
Pokemon Name: Serperior
Moveset Name: SubSeed Tormentor
Move 1: Substitute
Move 2: Leech Seed
Move 3: Torment
Move 4: Protect
Item: Leftovers
Ability: Overgrow
Nature: Timid
EVs: Multiple Spreads to use:
148 Def / 144 SpDef / 216 Spd [To outspeed max Spd +Spe nature Cobalion]
160 Def / 156 SpDef / 192 Spd [To outspeed max Spd +Spe nature Mismagius]
180 Def / 176 SpDef / 152 Spd [To outspeed max Spd +Spe nature Victini/Flygon]
The point of this set is to capitalise on Serperior's near unique access to the Subseed combo and Torment (The only other Pokémon getting this being Shiftry, who is slower outside of the sun, and with drought gone that isn't a factor, and frailer) to get easy subs. Substitute and Leech Seed are there for obvious reasons, as it wouldn't be a Subseeder without them. Now for the crux of the set, Torment. Torment can force a lot of switches, as Pokémon such as Heatran have proved in the past. It can also make getting subs much easier, and works even better in tandem with Protect. With Protect, you can simply block their attempts at breaking your sub with any super effective attacks or STAB moves, and then just do whatever you want to on the turn while they can't use any sub-breaking moves, and if they can, just sub again. If you do land a Leech Seed on them, after the turn of subbing up again and then protecting, you'll have only lost 1% of your health by the end of it thanks to Leftovers, making it a very good stalling Pokémon in the right hands.
The EV spread you should use vary depending on what you want to outspeed and how much bulk you want. Cobalion is the fastest thing of note in the UU tier that doesn't outspeed Serperior with it's maximum possible speed, however, if you don't feel the need to outspeed Cobalion, you can invest less speed and increase your bulk, and I left some notable speed points on the spreads. There isn't a point in going any slower than guaranteeing that you outspeed base 100 speed Pokémon, as at that point, you might has well be using Celebi. The EVs are put into the defenses instead of HP for a higher HP recovery from Leech Seed.
This set has uses over Subseeders such as Celebi, as while Serperior is less bulky than it's onion headed rival for a team slot, it doesn't have to invest as much into speed to get the first move on a lot of Pokémon, so it can afford to put more EV investment into it's bulk. The extra speed is also beneficial. Because of the Torment and Protect combo, Serperior also has a much easier time keeping it's Substitute up as well.
This Serperior can force a lot of switches, so having something else on the team with entry hazards can cause the opponent's team to get whittled down by residual damage a lot faster. Having something to wall Pokémon such as Scarf Heracross, and something to take random Ice Shard users is also handy as well.