Nair is Ness' most useful aerial. Bair is amazing too, but it shouldn't be used too often because then it becomes stale and it loses its excellent kill potential.
Ftilt is underrated, sure. But it's significantly shorter ranged and slower than the average ftilt. It makes up for this though by having amazing priority (it clanks with strong moves like Wario's fsmash) and decent knockback. Ftilt kills are possible at around 120%+ if you sneak one in near a ledge. Also being slow isn't necesarily bad, I've hit people out of spotdodges with it.