I like psycho boost on my lugia. Give it protect to waste the opponent's pp with pressure, then recover to restore health, and Aeroblast for super strong flying STAB move, and Psycho Boost for uber strong Psychic power! Give it a white herb and it restores its stats. But because of Lugia's good defensive capabilities, you don't need to worry about damaging the opponent when they're out of pp.
And I'm like the tenth person to say this, but a purified Lugia from XD knows Psycho Boost, Earthquake, Featherdance, and Hydro pump. Sounds like a moveset a 6 year old would give it. >_>