| Rank | Pokemon ---------| Usage % --| Raw ---| % -------| Real ---| % ------|
| 1 ----| Landorus-Therian | 38.62704% | 983149 | 26.233% | 839032 | 29.471% |
It is to no one's (or maybe someone's) surprise that Landorus-T is the most used (and most powerful/useful) in OU, and is rightfully S-Tier, the highest a Pokémon can be rating in any of Smogon's Metagames.
Why? Landorus-T can run many different roles effectively, for example, it can be an Offensive or Bulky Pivot, can set Stealth Rock or remove hazards with Defog, has a lot of useful resistances, has Knock-Off to hinder item dependent Pokémon, uses Intimidate to solidly switch into most physical threats, and even be a great Physical Wallbreaker with its access to powerful Z-Moves and Sword's Dance.
Landorus-T, like any Pokémon, is not perfect, and can be stopped by faster threats such as Tapu Lele and Mega Medicham, Ice and Water type attacks, Pokémon immune to Ground, and bulky Grass Types.
However, these flaws are out-shined by Landorus's perks, and Landorus-T's teammates often times take care of the threats it cannot deal with itself quite easily.
Landorus-T fits into the Meta game as one of the key Pokémon the Meta is based around, is used in nearly 40% of OU battles, and is unlikely to move from its throne for quite a while. :P
Other strong Pokémon by usage are: Magearna, Greninja-Ash, Heatran, Zygarde, Toxapex, Zapdos, Tornadus-Therian, Chansey, and Kartana, but a lot of other Pokémon in OU are strong as well. :P
Sources: Usage Stats, Viability Rankings
Hope I Helped!