Kingdra

Kingdra is unfortunately disallowed in UU but on the fringes of viability in OU. As a bulky Water-type, it faces stiff competition from the likes of Suicune, Vaporeon, and Starmie, all of which have useful qualities that Kingdra lacks—Suicune is substantially bulkier across the board; Vaporeon has Growth; and Starmie has Recover, Rapid Spin, Thunder Wave, and useful resistances to Fighting and Psychic. Nevertheless, Kingdra can be surprisingly effective in circumstances where these other Water-types usually struggle. Its Dragon typing relieves it of all relevant weaknesses, meaning that it's harder for the nearly ever-present Electric-types to force out, and it can neatly keep very tricky mixed attackers under control, such as Nidoking, Gengar, and Tyranitar. These unique defensive qualities, its decent damage output, and its status threat justify its use as a catch-all check that can trade hits adequately with almost any foe other than Snorlax.