Just to clarify, this means how good they are in the lead position, not how good a pokemon that can lead happens to be in general. This is just my list, but feel free to post your own, or tell me what you think is wrong with mine. Note that the pokemon aren't ranked in order within a tier.
S
Alakazam
Jynx
Starmie
A
Exeggutor
Hypno
B
Gengar
Venusaur
Victreebel
C
Articuno
Moltres
D
Everything else
Analysis
S: Alakazam is the best sleep absorber in the game and doesn't have to switch out against any other common lead. The exception would be if the enemy lead has paralysis and you know they have a wrap team. This is probably the only time where you'd rather have your zam take sleep instead of paralysis. Jynx is the next best absorber, but has the perk of doubling up as a sleeper as well. Starmie is an inferior absorber to the previous two due to not being able to sleep on chansey, but can hit many common leads harder depending on what moves it runs and is considered an overall better pokemon that jynx.
A: Still solid leads, but inferior to S for one or two reasons. Eggy is a great pokemon in general, but not a good asborber. Hypno is a good absorber who doubles up as a sleeper, but is otherwise not that good of a pokemon. Leading with a pokemon that is good overall but a bad absorber is sub-optimal. This is because it means it either takes sleep and you get a bad or even sleep trade (if they also let a bad asborber take sleep) at best, or you're forced to switch when you could've just lead with an S pokemon and not wasted that turn switching.
B:Generally bad leads that can get a good trade if lucky. Those pokemon risk being OHKOd by crits just to land a sleep, which is not that big of a reward, especially if they sleep a good absorber. The main reason for leading one of the two grasses is to have a grass in your team that outspeeds eggy for the sleep. If wrap is allowed, bel is a poor lead but is a good overall pokemon.
C: Bad leads that probably shouldn't be used, but are used on rare occasions because they counter eggy. Moltres is in general an inferior pokemon to articuno (although it does have agility+fire spin), though it threatens jynx as well as eggy. These are bad leads because they are countered by switching and are otherwise not great pokemon. People who try this usually either value getting the first sleep, or are just bad.
D: Basically other random stuff that people do.
S
Alakazam
Jynx
Starmie
A
Exeggutor
Hypno
B
Gengar
Venusaur
Victreebel
C
Articuno
Moltres
D
Everything else
Analysis
S: Alakazam is the best sleep absorber in the game and doesn't have to switch out against any other common lead. The exception would be if the enemy lead has paralysis and you know they have a wrap team. This is probably the only time where you'd rather have your zam take sleep instead of paralysis. Jynx is the next best absorber, but has the perk of doubling up as a sleeper as well. Starmie is an inferior absorber to the previous two due to not being able to sleep on chansey, but can hit many common leads harder depending on what moves it runs and is considered an overall better pokemon that jynx.
A: Still solid leads, but inferior to S for one or two reasons. Eggy is a great pokemon in general, but not a good asborber. Hypno is a good absorber who doubles up as a sleeper, but is otherwise not that good of a pokemon. Leading with a pokemon that is good overall but a bad absorber is sub-optimal. This is because it means it either takes sleep and you get a bad or even sleep trade (if they also let a bad asborber take sleep) at best, or you're forced to switch when you could've just lead with an S pokemon and not wasted that turn switching.
B:Generally bad leads that can get a good trade if lucky. Those pokemon risk being OHKOd by crits just to land a sleep, which is not that big of a reward, especially if they sleep a good absorber. The main reason for leading one of the two grasses is to have a grass in your team that outspeeds eggy for the sleep. If wrap is allowed, bel is a poor lead but is a good overall pokemon.
C: Bad leads that probably shouldn't be used, but are used on rare occasions because they counter eggy. Moltres is in general an inferior pokemon to articuno (although it does have agility+fire spin), though it threatens jynx as well as eggy. These are bad leads because they are countered by switching and are otherwise not great pokemon. People who try this usually either value getting the first sleep, or are just bad.
D: Basically other random stuff that people do.