Vaporeon

Despite its excellent HP stat and and access to Hydration + Rest, Vaporeon is a thoroughly terrible Pokemon in BW OU. As an offensive Water-type, it is completely shut down by Ferrothorn and is forced out by faster Pokemon like Latios, Alakazam, Thundurus-T, and Starmie. Keldeo and Starmie are better choices, as the former can break past special walls like Chansey and Ferrothorn with Secret Sword, while Starmie has better natural Speed, Electric coverage to hit Politoed and Tentacruel, and Analytic, which further punishes switch-ins. Vaporeon is also outclassed as a defensive Water-type by Pokemon like Rotom-W, Jellicent, Gastrodon, and Slowking as a result of their better typings and longevity letting them check more prevalent threats, including Keldeo, Starmie, Thundurus-T, and Alakazam. Other Water-resistant Pokemon like Ferrothorn, Latios, and Celebi outclass Vaporeon as well, as they have better overall typings, offensive utility, and movepools, and they function outside of rain, making them fit on many more teams than Vaporeon.