Exeggutor

With a very high Special Attack stat, solid STAB attacks, dangerous status moves, and a powerful Explosion, Exeggutor is easily one of the most threatening Pokemon in GSC OU. While it will rarely sweep a team by itself, it often plays a key role in dismantling the opposing team's defenses through its ability to incapacitate or trade for crucial targets. On the defensive side, Exeggutor can serve as a check to threatening sweepers like Vaporeon, Marowak, and Machamp thanks to its resistances to their powerful STAB attacks. Unfortunately for Exeggutor, a lot of OU Pokemon run coverage moves that hit it hard, so you should time its entry onto the battlefield carefully to ensure it isn't forced to use Explosion too quickly.