Tyranitar

Tyranitar is one of the most definitive Pokemon in BW OU thanks to it setting sand and keeping dangerous Psychic-types like Latios, Reuniclus, and Alakazam in check by Pursuit trapping them. Sand is a key component of the metagame, since it keeps rain off the field, limiting dangerous Pokemon on rain teams like Thundurus-T and Tentacruel, and works very well alongside other means of residual damage, like entry hazards and status. Sand damage puts Pokemon like Keldeo, Latios, Rotom-W, and Starmie on a timer and negates Leftovers recovery for many others. Outside of Psychic-types, Tyranitar also checks common threats like Thundurus-T, Heatran, Jellicent, and Volcarona if running Tanga Berry, and it can revenge kill and Pursuit trap faster threats with a Choice Scarf set. Tyranitar can even run lure options for checks like Skarmory, Excadrill, and Gliscor thanks to its deep movepool. However, Tyranitar is quite vulnerable to being worn down by the Pokemon it should check, given its lack of recovery and vulnerability to entry hazards. Most noticeably, opposing sand teams can use multiple Psychic-types to overwhelm Tyranitar in tandem with entry hazards. It also has common weaknesses that leave it forced out by Pokemon like Keldeo, Excadrill, Terrakion, and Scizor and 2HKOed by Focus Blast Reuniclus and Alakazam, which Tyranitar would otherwise Pursuit trap without much issue. It's also slow, making its Choice Scarf set fail to revenge kill any boosted threats or outspeed and beat Alakazam.