Gengar

As the only fully evolved Ghost-type Pokemon in RBY, Gengar fulfills a unique role that no other Pokemon in the generation manages to accomplish; its immunity to Normal-type moves means that it does not take damage from Body Slam, Hyper Beam, Explosion, and Self-Destruct, four moves that completely shape the tier. Gengar is also the fastest sleep inducer available, getting the jump on other sleepers such as Jynx and Exeggutor, an extremely strong trait in a generation where the sleep status is so centralizing. Finally, its high Special and Speed mean that it is capable of dishing out strong attacks that come with a 21.48% critical hit rate attached to them. This high critical hit rate also comes into relevance when Gengar uses Explosion, turning it into a very dangerous and disruptive Pokemon.

However, Gengar also comes with an array of shortcomings. First, its Poison typing brings virtually no upsides and gives Gengar two very crippling weaknesses to Earthquake and Psychic, two of the most common attacking moves in the tier. Most notably, Tauros and Snorlax 2HKO Gengar with Earthquake, and Starmie, Exeggutor, Alakazam, and Jynx 2HKO it with Psychic, severely hindering Gengar's ability to take on these Pokemon. Second, the two most important moves in its arsenal both come with a caveat: Hypnosis has a very shaky 60% accuracy that will often prove extremely crippling when it misses, and Explosion comes off of a mediocre Attack stat, meaning that barring critical hits, it is one of the weakest Explosions in the tier. Third, since its attacking options do not benefit from a STAB boost, Gengar's damage output can oftentimes prove to be underwhelming. Lastly, despite being by no means a slow Pokemon, Gengar's Speed can still leave much to be desired, as it Speed ties with Tauros and is outsped by Starmie and Alakazam.