Starmie

Starmie is one of the most common Pokemon in RBY OU thanks to its Psychic typing and access to Recover and Thunder Wave. Starmie's physical bulk allows it to be a defensive check to virtually every Pokemon lacking an Electric-type attack and take two hits from almost any physical attacker. Additionally, its access to many special attacks such as Blizzard, Psychic, and Thunderbolt turns it into an offensive threat versus anything without instant recovery. Although Starmie naturally outspeeds many common Pokemon, most notably Tauros, oftentimes a paralyzed Starmie functions well in its defensive role due to its ability to block sleep-inducing moves from almost anything, while the ability to paralyze any non-Ground-type Pokemon via Thunder Wave makes it difficult to pressure offensively even then. Among the Recover and Soft-Boiled users in OU, Starmie also possesses a special defensive utility, as it's the only one of them to not be forced out immediately by Snorlax when paralyzed and is able to fight back against it offensively as well when running a STAB move. Those traits make Starmie usable in pretty much any state of the game, turning it into a very flexible Pokemon and easy addition to most RBY OU teams.