I would like to nominate
Weavile.
Weavile @ Focus Sash
Ability: Pressure
Level: 50
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Fake Out
- Icicle Crash
- Knock Off
- Feint/Swagger
Weavile provides the team with valuable fake out support, creating opportunities for our restricteds to dish out damage, for Cresselia to use speed control, etc. Weavile is particularly good at this due to its base 125 speed, which allows it to outspeed almost any opposing fake out or quick guard user (notably adamant Talonflame). Icicle Crash does huge damage to ice-weak targets like Thundurus, Crobat, and opposing Yveltal, plus it comes with a 30% flinch chance. Knock off is a strong STAB move which also gets powered up by Yveltal's Dark Aura, and removing items can be quite useful. The 4th slot is extremely flexible, the options I listed are just the two that I think would work best here. Correctly predicting with Feint can drastically swing the game into your favor, and forces your opponent to think twice about making defensive plays. As Bright Size mentioned Swagger combos nicely with Yveltal Foul Play, and of course Swagger is always God. The other main options for a 4th move are Taunt, Low Kick, and Protect.
A few calcs:
252 Atk Weavile Icicle Crash vs. 248 HP / 36 Def Thundurus: 174-206 (94 - 111.3%) -- 68.8% chance to OHKO
252 Atk Weavile Icicle Crash vs. 4 HP / 0 Def Yveltal: 144-170 (71.2 - 84.1%)
252 Atk Dark Aura Weavile Knock Off (97.5 BP) vs. 36 HP / 204 Def Raichu: 126-148 (90 - 105.7%) -- 37.5% chance to OHKO
Swagger Support -
+2 252 Atk Life Orb Dark Aura Yveltal Foul Play vs. 204 HP / 204 Def Primal Kyogre: 188-224 (93.5 - 111.4%) -- 62.5% chance to OHKO