PK Fire, aka free grab button (need I mention one of the most powerful throws in the game?)
PK Thunder, a great zoning tool, on top of a kill move should you be able to hit yourself into someone.
Some absurdly powerful ariels.
His side smash is no slouch, either.
Really Ness' only huge weak point is a pretty easy to gimp recovery, where his double jump and an air-dodge frequently get the job done.