Smeargle

Access to nearly every move in the game creates an infinitely dangerous Pokemon, even if that Pokemon has as pitiful stats as Smeargle. While it really can't make use of its expansive offensive movepool, it can most definitely use its supportive movepool. Due to its lack of defenses and decent Speed, Smeargle is almost always used as a lead, even in the age of Team Preview. This way it can tank at least one hit thanks to Focus Sash and accomplish what it needs to. Spore is an almost universal move found on Smeargle thanks to it being the fastest user of the move in the tier. Beyond that, the possibilities are almost endless. Every setup move + Baton Pass, every entry hazard, dual screens, every supportive move, and every status move are at Smeargle's fingertips.

Unfortunately for Smeargle, sleep inducing moves are pretty common in RU, so almost every team is prepared for its Spore in some way or another. Absolutely zero offensive presence also means Smeargle is highly susceptible to Taunt, Substitute, and other setup moves, particularly if it's not carrying Whirlwind. Still, Smeargle will almost always accomplish its goal and is a very real threat in the tier.