I'm sorry I'm constantly bombarding this thread with replies, but I got another idea for a core in UU.
Victini @ Life Orb / Expert Belt
Ability: Victory Star
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Blue Flare
- Focus Blast / Psyshock
- Thunder / Thunderbolt
- Glaciate
Whimsicott @ Leftovers
Ability: Prankster
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Stun Spore
- Moonblast
- Encore
- Leech Seed
Empoleon
Leftovers
Ability: Torrent
EVs: 248 HP / 44 Def / 216 SpD
Calm Nature
- Scald
- Defog
- Stealth Rock
- Roar
These three work fairly well together. Victini runs the rather surprising Special Attacking set, which catches plenty of things off guard. While the choice between power vs. accuracy is fairly obvious in Thunder and Thunderbolt, I'm having a hard time deciding between Psyshock for Chansey, (Even though it still does only 30%) and Focus Blast for... other things. Is Focus Blast even relevant, seeing as Tyranitar isn't even a problem in UU? Anyways, Glaciate takes care of the newly arrived Dragons in Haxorus and Hydreigon, as Glaciate negates their Choice Scarf boost, and does a significant amount of damage to them. Next we have Whimsicott, returning as the annoying puffball that she was in Gen V. Stun Spore, Encore, and Leech Seed, backed by Prankster, allow her to safely shut down any sweeper hoping to set up on the rest of the team. Moonblast, meanwhile, progressively chips away at the opponent, and has great coverage compared to Giga Drain. In addition, Whimsicott covers Victini's weaknesses to Water, Dark, and Ground, while Victini can cover her weaknesses to Fire, Ice, an Steel. Finally, there's Empoleon, who serves as the Defogger and Entry Hazard setter. Empoleon runs the more Specially Defensive set, to coincide with Whimsicott's Physical Defense, and his type synergy works very well with the rest of the core. The three weaknesses Empoleon has to Fighting, Ground, and Electric, are covered by Whimsicott, while Empoleon covers Whimsicott's weaknesses to Steel, Flying, Poison, and Ice, as well as taking care of Victini's weaknesses to Rock and Water. That Steel Nerf kinda hurts this core though, as Ghost-types can do some serious damage, even to Empoleon. Regardless, I'm pretty satisfied with this core. What do you guys think?
EDIT: I've had a bit of time to think about some powerful UU Fighting-Types that can deal with Chansey. Life Orb Keldeo 2HKOes with Secret Sword, and has good synergy with the rest of the core. Heracross also works well, being able to punch through Chansey with a Guts Close Combat. Mega Medicham can also 2HKO, but that increases my Ghost-type weakness. I was just about to consider Terrakion as well, but he's BL now.