Aldaron
geriatric
Well, Celebi has above average attacking stats...above average speed...and Recover. A Life Orb set should be in the analysis. Gouki and I can both attest to its effectiveness. After discussing it with Gouki, we decided that Timid was the consensus "best" ability, as the extra power of Modest wasn't as vital as always outspeeding Lucario, Roserade, and Adamant Garchomp.
http://www.smogon.com/dp/pokemon/celebi
[SET]
Name: Sweeping Celebi
Move 1: Recover
Move 2: Leaf Storm
Move 3: Psychic
Move 4: Hidden Power Fire / Hidden Power Ground / U-turn
Item: Life Orb
IVs: 6 Atk / 30 SpA / 30 Spe
EVs: 36 Def / 252 SpA / 220 Spe
Nature: Timid
[SET COMMENTS]
<p>This Celebi's purpose is to hurt as much of the standard metagame as possible. Only Heatran and Houndoom resist the Hidden Power Fire set. Recover is on the set to heal off residual Life Orb recoil damage, as Celebi will have many opportunities to heal in battle due to its useful Fighting and Ground resists. Hidden Power Ground can be used to deal damage to Fire Pokemon switching in on Celebi, like Heatran, Houndoom, Infernape and Arcanine. U-turn can be considered in order to save Celebi from potentially bad matchups, and it works well in conjunction with Leaf Storm due to the -2 Special Attack drop.</p>
<p>Timid is chosen over Modest because it is impossible to have a Speed IV of 31 with Hidden Power Fire, meaning Celebi would always be slower than other neutral base 100 Speed Pokemon. 319 Speed allows Celebi to always outspeed and KO Lucario, Roserade, Porygon-Z, and Electivire. It also allows Celebi to deal massive damage to Adamant (Yache Berry) Garchomp (who would otherwise outspeed a Modest Celebi) with Leaf Storm.</p>
<p>Life Orb is the optimal item on this set instead of Expert Belt because Celebi will hit most of its targets for neutral damage. As for Leftovers, this particular Celebi set is not meant to take hits anyway.</p>
http://www.smogon.com/dp/pokemon/celebi
[SET]
Name: Sweeping Celebi
Move 1: Recover
Move 2: Leaf Storm
Move 3: Psychic
Move 4: Hidden Power Fire / Hidden Power Ground / U-turn
Item: Life Orb
IVs: 6 Atk / 30 SpA / 30 Spe
EVs: 36 Def / 252 SpA / 220 Spe
Nature: Timid
[SET COMMENTS]
<p>This Celebi's purpose is to hurt as much of the standard metagame as possible. Only Heatran and Houndoom resist the Hidden Power Fire set. Recover is on the set to heal off residual Life Orb recoil damage, as Celebi will have many opportunities to heal in battle due to its useful Fighting and Ground resists. Hidden Power Ground can be used to deal damage to Fire Pokemon switching in on Celebi, like Heatran, Houndoom, Infernape and Arcanine. U-turn can be considered in order to save Celebi from potentially bad matchups, and it works well in conjunction with Leaf Storm due to the -2 Special Attack drop.</p>
<p>Timid is chosen over Modest because it is impossible to have a Speed IV of 31 with Hidden Power Fire, meaning Celebi would always be slower than other neutral base 100 Speed Pokemon. 319 Speed allows Celebi to always outspeed and KO Lucario, Roserade, Porygon-Z, and Electivire. It also allows Celebi to deal massive damage to Adamant (Yache Berry) Garchomp (who would otherwise outspeed a Modest Celebi) with Leaf Storm.</p>
<p>Life Orb is the optimal item on this set instead of Expert Belt because Celebi will hit most of its targets for neutral damage. As for Leftovers, this particular Celebi set is not meant to take hits anyway.</p>