Celebi

Type Tier
Psychic / Grass Uber
Statistics
Stat
HP
100
403
Atk
100
298
Def
100
298
SpA
100
298
SpD
100
298
Spe
100
298
Name Item

Back to the Basics

Leftovers
Moveset
~ Heal Bell
~ Recover
~ Leech Seed
~ Psychic

This is a support set and pretty much all that Celebi has going for it in GSC. Heal Bell helps the rest of the team, Recover helps Celebi itself, Psychic gets STAB and uses its good Special Attack, and Leech Seed can force switches.

Other Options

Perish Song can act as a pseudo-phazing move. Celebi can just Recover until the three rounds are up. It pulls the Perish Song job very well with its good defenses. It also allows a stall team to not worry about a last-Pokémon Curser sweep. Giga Drain is a good move but frankly it isn't advisable. Celebi already has a recovery move, and unlike Exeggutor, Celebi is more defensive and should be played as such. An interesting option you could try is Curse coupled with Baton Pass, but it might leave you in a bind when Houndoom shows up.

Opinion

Great stats and resistance to three of the most popular attack types in the game is never bad (Water / Electric / Ground). One thing Celebi doesn't get its due credit for is being able to stall out the average Snorlax and force it to Rest without taking much damage thanks to Leech Seed. It also learns Heal Bell, which is one of the most valuable moves for a stall team, and is still useful for others.

Counters

Houndoom and Tyranitar are really the only things that will take it down with certainty. Espeon can Substitute and Growth before passing to something with Flamethrower. Zapdos has a chance to stop it with Drill Peck. Jynx has a good chance too, thanks to the double threat of STAB Ice Beam and Lovely Kiss, and its low HP means Celebi won't regain much with Leech Seed. If Celebi is paralyzed, Heracross and Pinsir make good counters. Charizard may be able to set up a Belly Drum on Celebi, or just nail it with STAB Fire Blast. Really though, this thing can be hard to take out.