Okay, since everybody here except for DD seems to not understand the point of Celebi, I am going to answer this even though I have only used Celebi in Shoddy Battle.
Okay, there are two main movesets you can use for Celebi.
This moveset makes use of Baton Pass, mainly Swords Dance or Nasty Plot. It can also use Grass Knot or Seed Bomb+Thunder Wave in order to do some damage of its own or make things easier for it.
This is a normal set. Thunder Wave, Leaf Storm/Grass Knot, U-Turn, and Trick all work well together. In fact, you can use Trick with Toxic or Flame Orb, U-Turn and Natural Cure removes the status. Celebi also uses Leaf Storm well as it switches out so often. Thunder Wave is for any Pokemon after the Pokemon you tricked.
There are some other good options though. Hidden Power Fire can be used to deal with Steel types, and Hidden Power Ice can deal with Dragons and Flying types. Earth Power deals with Fire types and Steel types, while Perish Song is good Psuedo-Hazing. Calm mind is an option for Baton Passing. Energy Ball can be used for reliable STAB. Psychic also provides STAB. Recover is also a good option.