Alakazam is one of the most reliable leads in RBY, because of its blistering 120 Speed which makes it the fastest lead in the tier outside of Jolteon. This means Alakazam is one of the fastest Paralysis spreaders in the tier, outspeeding everything up to opposing Alakazam and Jolteon. Alakazam also has a great Special stat, which lets it take hits from opposing leads such as Jynx, Exeggutor, and Starmie well which lets it retaliate with Thunder Wave and proceed to weaken them with Seismic Toss or force them out with Special drops from Psychic. Because of its great Special bulk and resistance to Psychic, Alakazam is one of the most reliable sleep absorbers in the tier as it can safely pivot into the likes of Jynx, Exeggutor, and Gengar, in order to try gain momentum. Alakazam is also the safest lead against Gengar, as it can OHKO with a critical hit which happens a staggering 23% of the time, and Gengar also needs to hit a 60% accuracy move should it avoid being hit by a critical hit.
Starmie is very similar to Alakazam in that it's a fast Psychic-type lead with access to Recover and Thunder Wave and solid bulk. What mostly separates it from Alakazam is its larger offensive movepool which includes Blizzard and Thunderbolt, Blizzard in particular is useful because it deals very heavy damage to Exeggutor and it can threaten the likes of Chansey with a freeze, meanwhile it can deal solid damage to opposing Starmie leads with Thunderbolt. However, Starmie lacks a set damage move in Seismic Toss, meaning that it will usually struggle and lose to opposing Alakazam leads.
Jynx is one of the best, if not, the best Sleep lead in RBY. It has access to Lovely Kiss which has considerably higher accuracy than Hypnosis (75% compared to 60%), it also has a useful pair of resistances to Psychic and Ice, which means it can get off a Lovely Kiss against opposing not using Gengar easily. Because of its nifty resistances to Psychic and Ice, two of the most common offensive types in the metagame, Jynx usually handles being put to sleep well, living hits from the likes of Chansey and Starmie easily until it wakes up. Jynx also has a decent 95 Special, but is very difficult to switch into after something has been put to sleep as the threat of a freeze from Blizzard threatens the likes of Chansey, Starmie, and Slowbro meanwhile Ice-types such as Cloyster, Articuno, and Lapras do not want to switch into Jynx's Psychic STAB.
Gengar is considered fairly risky to use in RBY as its only Sleep inducing move has a horrible 60% accuracy, and on top of that, it has an awful lead matchup against Alakazam and Starmie, both of which can OHKO with a critical hit Psychic. However, Gengar's Ghost typing is unique to it in RBY, so it makes for a solid status absorber as its presence can deter the likes of Exeggutor and Snorlax from using Explosion / Self-Destruct, which can cost your opponent the game if they potentially throw away their boomers. Gengar itself also has access to Explosion, which makes it a very powerful presence late-game if it has woken up or the opposing team is also paralyzed, which means it can pick off anything as long as it has been weakened enough for Explosion to OHKO, which usually isn't very hard. Gengar usually has a solid matchup against anything slower, but faster Pokemon can be difficult for Gengar to handle.
There are other leads in the RBY OU metagame which can be used, including Exeggutor, which has a solid chance against Alakazam leads, but all other leads deal heavy damage, or outspeed Exeggutor and put it to sleep. Tauros also has potential as a lead, but is only good against Jynx leads, forcing them out early-game, but it has a bad matchup against any other lead. Ninetales can also be used as an anti-sleep lead, capable of taking on Jynx, Exeggutor, and Gengar, but has very little use outside of that due to its weakness to common Water- and Ground-types.