Gengar

Gengar possesses the best Speed of any sleeper in the game, a unique Normal immunity, and the coveted Explosion. It outpaces relevant threats like Jynx, Mew, and Zapdos, allowing it to cripple them with Hypnosis before they can threaten it in return. Gengar's Normal immunity carries substantial utility, letting it absorb the plentiful Explosions in the metagame for free and walling Mew and Snorlax sets carrying only Normal-type moves. Additionally, it can come in handy against Persian and Wrap users like Dragonite and Victreebel, though these Pokemon do not see much use. Gengar can use its own Explosion, armed with a fearsome 21.47% critical hit rate, to significantly damage anything that is not Rock- or Ghost-type, making problematic foes simpler to dispatch and enabling offensive teammates like Mewtwo to make progress. Aside from this, Gengar packs Night Shade to do decent, consistent damage to all Pokemon and a powerful Thunderbolt to threaten Water-types like Slowbro.

Gengar is hindered by its inconsistency—it has to rely on Hypnosis's low accuracy, and this is exacerbated by its poor physical bulk and unfortunate weaknesses to Psychic and Ground. Moreover, while Gengar is immune to Body Slam and thus its paralysis chance, other moves like Thunder Wave and Stun Spore can still cripple it, preventing it from outpacing naturally slower Pokemon and paralyzed Mewtwo and threatening them with Explosion. After Gengar gets off sleep, it can't do much to Rhydon and Golem unless it decides to drop a move for the otherwise useless Mega Drain, and it needs Explosion to threaten many other relevant Pokemon like Exeggutor, Chansey, Mew, and Mewtwo. These flaws do not outweigh Gengar's unique utilities, however, and it is a solid pick on an RBY Ubers team.