Yanmega has an excellent Special Attack stat, very respectable Speed, and two of the most powerful abilities in the game at his disposal. With Tinted Lens, Yanmega is capable of 2HKOing even dedicated walls who resist his STAB attacks, an impressive feat. Meanwhile, if he instead chooses to use Speed Boost, he will outspeed the entire metagame, including Choice Scarf users, after a turn or two.
It's hard to imagine Yanmega being anything other than top-tier OU after considering his strengths, yet the fact remains that he's not. Yanmega suffers from an utterly crippling 4x weakness to Stealth Rock that has completely nullified his presence as a standard of the metagame. Weaknesses to common Fire-, Ice-, and Electric-type attacks aren't helping Yanmega's position either. These weaknesses, coupled with Yanmega's not-so-stellar defenses, have sealed the dragonfly's fate and unfortunately banished him to the bottom of OU.
Yanmega's unique attributes allow him to function as an excellent anti-lead in the OU metagame. After a single Speed Boost, Yanmega will outspeed every single non-Choice Scarf lead bar Ninjask and hit them all for serious damage. Yanmega can also function excellently as a late-game sweeper if the need arises and he survives his early-game encounter.
Protect gives Yanmega the edge against Fake Out or attacks from faster leads, particularly from Infernape and Starmie, who both outspeed him before Speed Boost kicks in. As Protect also guarantees that Speed Boost will activate once and maintains Yanmega's Focus Sash, it enables Yanmega to subsequently beat both Starmie and Infernape. Bug Buzz and Air Slash are Yanmega's STAB attacks, and when used together, provide reasonable coverage against most common leads, only missing at least neutral damage against Heatran, Aerodactyl, and Skarmory. It is for this reason that Hidden Power Ground and Hidden Power Fire are the choices for the fourth slot. Hidden Power Ground is useful for 2HKOing standard Shuca Berry Heatran leads and for a greater likelihood of beating Metagross leads, given the presence of Occa Berry. Hidden Power Fire, however, allows Yanmega to defeat Skarmory, Forretress, and Abomasnow leads at the cost of being walled by Heatran. Hidden Power Fire also serves as a weapon with which to combat Scizor later in the match, who can only 2HKO with Bullet Punch. Hypnosis can be used in the fourth slot over Hidden Power for a better chance at beating Tyranitar who lack Lum Berry and Swampert, but it is terribly unreliable and generally not worth sacrificing the ability to beat other leads reliably.Team Options & Additional Comments >>>
With Tinted Lens, Yanmega's STABs give him perfect type coverage, hitting everyone in the game for at least neutral damage. Coming off of a base 116 Special Attack stat, even Pokemon who normally resist Yanmega's STABs, such as Skarmory, are reliably 2HKOed. There are truly very few safe switch-ins to this Yanmega, as he has the power to punch through practically anyone in the metagame.
Bug Buzz and Air Slash are mandatory on this set, giving Yanmega perfect type coverage within his STAB attacks alone. Hidden Power Ground is there to OHKO Pokemon who might try to switch in on Bug Buzz with their 4x resistance, such as Heatran and Lucario. U-turn fills the final slot perfectly, allowing Yanmega to scout the opponent's initial switch-in and bring out the necessary response. Be wary when using U-turn, however, as with Yanmega's Stealth Rock weakness, he cannot switch in haphazardly.Team Options & Additional Comments >>>
The listed sets already take advantage of Yanmega's most prominent movepool features, but there exist a few more that can be of use. Hidden Power Ice allows the LeadMega set to OHKO Lead Dragonite and can allow the Specs Tinted Lens set to OHKO Flygon switch-ins, but leaves both vulnerable to Heatran. Shadow Ball on the Specs Tinted Lens set will OHKO defensive Rotom-A, but Bug Buzz would do that after Stealth Rock damage anyway. As noted in the LeadMega analysis, Hypnosis can be run to deal with some otherwise troublesome Pokemon, though its accuracy is a letdown.
Checks and Counters
Outside of Blissey, absolutely nobody perfectly walls Specs Tinted Lens Yanmega. Skarmory and Heatran can come in on a predicted Bug Buzz and force him out to take more deadly Stealth Rock damage, but must be wary of a 2HKO from Air Slash. Snorlax is a good response with his monstrous special bulk, though Bug Buzz will still 3HKO him, an issue made worse by Snorlax's lack of reliable recovery. Anyone faster or with strong priority who can get in on an attack and survive will force Yanmega out. Scizor's Bullet Punch, Lucario's ExtremeSpeed, and Choice Scarf Heatran's Fire Blast immediately come to mind as suitable options for the job.
The biggest counter to Yanmega, however, is not any one Pokemon, but rather a battle condition. Stealth Rock completely ruins him, and with its prevalence in the metagame, just maintaining it is enough to keep Yanmega at bay.