I'm not wholly sure what the slashes should be on Nasty Plot celebi, but I'm certain not happy with the way they are now. Although I've only used NP Celebi in a few matches since the introduction of Genesect, prior to that introduction Hidden Power Ice or Psychic was, for me, a necessity on a Nasty Plot celebi set. Being walled by every Dragon and almost every Flying-type in OU is not remotely acceptable. For me, Hidden Power Ice was the single best coverage move Celebi could use - it hits Salamence, Dragonite, Hydreigon, Landorus, Latios, Latias, and opposing Celebi significantly harder than Psychic, while Psychic would hit most targets reasonably, and also provide a very nice hit on Amoonguss, Gengar and Conkeldurr.
Prior to the introduction of Genesect, I would have the slashes listed as follows:
Nasty Plot
Giga Drain
Hidden Power Ice / Hidden Power Fire
Earth Power / Psychic / Recover
Obviously, Genesect's introduction would rather change things, encouraging the use of Hidden Power Fire to OHKO it on the switch, rather than to deal around 50% with a Life Orb Earth Power and rather less with Leftovers. Speaking of items, you don't have one on that set at the moment. Leftovers / Life Orb for me.
edit: split the defensive sets please. Non-negotiable as said before.