Mew

Mew is one of the most threatening Pokemon in GSC Ubers thanks to its insane versatility that lets it perform an important variety of offensive roles. As a lead, Mew matches up perfectly against common leads like Snorlax and Mewtwo, consistently beating them with the combination of Swords Dance, Thunder Wave, and Explosion, with Miracle Berry further helping Mew against them by providing a one-time status immunity that lets it set up freely. Mew's Explosion is one of the strongest attacks in the game, capable of potentially breaking through even the bulkiest of Pokemon, like Lugia, Skarmory, Barrier Mewtwo, and Tyranitar after a Swords Dance boost, making Mew one of the best offensive facilitators for teammates like Snorlax, Tyranitar, and offensive Mewtwo.

When used in the back, Mew takes advantage of its good natural bulk to annoy offensive Mewtwo, Zapdos, and Gengar, matchups that can be further improved if Mew is running Soft-Boiled in its moveset. Mewtwo's Self-Destruct and Gengar's Explosion don't come close to OHKOing Mew, meaning that it can recover the damage done with Soft-Boiled or survive with enough HP to attack the next turn.

However, Mew can be reliant on paralysis to effectively remove Barrier Mewtwo; Mewtwo's higher Speed allows it to reach +4 Defense as Mew uses Swords Dance once, thus making Explosion's chances to OHKO it lower and lower as Mewtwo racks up more boosts. Mew's Speed stat also leaves it outsped by Jolteon and the aforementioned Mewtwo, two Pokemon that can reliably revenge kill a low-HP Mew. Finally, Mew, while having access to an ocean of a movepool, cannot beat every check to itself with four moves, and thus, if it's not sacrificing itself with Explosion, it becomes much less threatening once its entire moveset has been revealed.