Gengar's not top 5 in RBY. That'd probably be Tauros, Chansey, Exeggutor, Snorlax, and Alakazam. Gengar's high risk, high reward. It can absorb booms (and boom as well), but you've gotta be sure that it's taking that Explosion. Predicting wrong could mean eating Stun Spore or Psychic from Exeggutor, Earthquake from Snorlax, or an EQ from Golem that usually OHKOes. Hypnosis only has 60% accuracy, so 40% of the time it'll risk getting hit with sleep or a strong attack itself. Using it in the lead slot will also make it lose the surprise factor of absorbing Explosions, and your opponent will play with Gengar in mind.
It's not bad, though. It's immune to Wrap damage, making it the safest stop to Dragonite sweeps. It'll be unable to move after getting Wrapped, however, so your opponent still gets a free switch to a counter. It has the same Speed as Tauros, so it also gets that sweet 21.5% crit chance, which is worth mentioning. You /need/ to keep it safe from paralysis, though, cuz its EQ + Psychic weakness makes it easy to take out if it's not faster than things.
- 21.5% crit chance
- Above-average Speed
- Wrap damage immunity
- (a weak) Explosion
- Explosion immunity
- 1-on-1 and after sleep clause, it usually beats Dragonite (has to Explode if it already set up Agility), Chansey (has to Explode), Lapras, Starmie without Psychic, and Jynx (has to Explode)
- Hypnosis's accuracy
- Limited movepool (it's not particularly powerful tbh)
- Earthquake weakness
- Psychic weakness
- 1-on-1 after sleep clause, it usually loses to Tauros, Snorlax, Golem, Rhyhorn, Exeggutor, Starmie with Psychic, and Zapdos
It has a slight edge against Alakazam if it Explodes (counting crit rates for both, it's about a 56% chance to win), and can beat Slowbro that hasn't set up Amnesia yet (though it'll be eating a Thunder Wave, and an already +1 Slowbro is a different story, especially if it has Psychic.)