What to use:
Yanmega @ Life Orb
Trait: Speed Boost
Modest Nature (+SpA, -Atk)
Evs: 4 HP / 252 SpA / 252 Spe
-Air Slash
-Bug Buzz
-Hidden Power [Ground / Ice]
-Protect
Role: Special Sweeper, Late-game Cleaner,
What It Does: Yanmega is a really underrated pokemon that has to compete with the likes of Volcorona. However this doesn't mean it's not good anymore. Yanmega have access to a ability known as speed boost. Each and every turn, it get's +1 speed making it a very threatening sweeper as eventually, it can outspeed everything on your team and if you're not prepared, gg. Air Slash is a great stab move, hitting a lot of things for at least netural damage if not super effective. Bug Buzz is a stronger stab move to use in situation where you're not hitting hard enough. Hidden Power Ground destroys Heatran's (if they don't have a air balloon) and Magnezone's. Hidden Power Ice kills dragons like Salamence, and Dragonite. It also can be used to kill Landorus. It's up to what you want to hit. Choose wisely. Protect gives you a free speed boost to outspeed things you normally can't. Choice Scarfers? At +2 Yanmega outspeeds scarf Latios with Modest. It basically doesn't need timid so no need to worry.
Good Teammates: Steel types like Ferrothorn are Yanmega's archenemy. You would want a Magnezone to trap these fuckers that prevent you from doing major damage to the opponents team. Special walls such as Chansey, Blissey, and Jellicent also stops Yanmega. A Weavile makes a good partner as it traps Jellicent, and kills the fat blobs with Low kick. Not to mention Mamoswine makes a great check to Yanmega as it can just hit it with Ice Shard. Weavile fits this role greatly as well.
What Counters It: Chansey, Blissey, Jellicent, Ferrothorn can all take hits from it for days and cripple Yanmega from doing it's job. Rotom-W can take one hit from it and dispatch it with Volt Switch. Priority like Bullet Punch and Ice Shard easily brings Yanmega down.
Any Additional Info: Use this thing before saying it's bad. It's not. It's actually great at doing it's job. Try it
What not to use:
Alomomola @ Leftovers
Impish Nature (+Def, -SpA)
Trait: Regenerator
Evs: 252 HP / 252 Def / 4 SpD
-Waterfall / Soak / Scald
-Toxic
-Wish
-Protect
Role: Physical Wall
What It Does: Why the hell would you want to use this thing? Seriously? All it got that's useful is the moves that are listed above. It's movepool is so bad that it's almost always the same. The only time when it's different is when people use Scald instead of Waterfall. In fact I actually heard some people said this thing kicks ass in OU and it's underrated. No it's not. What you're going to do with this thing? Take physical hits? Out of all the bad ones you pick this piece of shit. The only thing going for it is it's massive hp and good def. You can use Slowbro. There are a lot of better bulky water types that are so much better. Don't tell me you're going to spam Shadow Ball with it. Or even maybe Return with a fucking choice band. Does nothing but bored your opponent to death with Wish + Protect while it does like 5-10% damage.
Good Teammates: If you're going to insist on using this thing, get out of my face. I have no reason to argue with you if you're this ignorant. But I'll list the good teammates anyway. Pick something that forms a good core with this thing. Celebi, Amoonguss, Heatran and stuff like that. If you want, you can use it as death fodder for pokemon's that locks themselves into a Electric move so you can get a free Nasty Plot / Agility with Thunderus-T.
What Counters It: Anything that outstall and wall it. All the good grass types like Celebi, Amoonguss, Virizon makes it cry. All the other bulky water outstall it. Rotom-W does a fuckton to this thing with Volt Switch. Used as setup fodder for a lot of pokemon: Thunderus-t, Salamence, Dragonite, Garchomp, Jirachi, Virizion, Celebi, Terrakion, Gyarados, Keldeo, Kyruem-B, Latias, Lucario, Reuniclus, Toxicroak, Venusaur, Volcorona, Haxorus, Conkeldurr, Cloyster, Breloom. That's literally almost the whole entire OU metagame, expect that Virizon is in UU. I just putted there because I feel like I should mention it.
Any Additional Info: Don't use this thing EVER. If someone tells you this thing is good in Ou, just ignore them.