Ness is awesome all his attacks are very powerful, especially his air attacks :
His B-air is soooo powerful, It is by far his most powerful Non-Special attack because hum.... (*cough* PK Pulse *cough*)
BUT you have to connect it properly, you have to hit B-air half-a-second before touching the enemy, you know when you did connect it properly because him an the enemy freeze for like 0.3 seconds in the air and the enemy is "shocked".
His F-air is just as good, It can deal multiple hits and decent %. But you really take advantage of it when you do it while falling on the ground, meaning you have to hit the buttons like 1 second before touching the ground, because when you do that the enemy gets pushed back a little and can't counter.
His D-air is one of the most powerful spikes and attacks too.
His U-air is one heck of a headbutt, but is really not that good when someone has low pourcentage because they don't get pushed too far up and consequently they can counter easily, so only use U-air at about 50% or more.
Now his PK fire, a lot of people spam it, A LOT (i too tend to use it, close to spamming) BUT it is nonetheless one good attack, especially on the BIG, HEAVY, chars *cough* Bowser *cough*) example .: Bowser is the best example because once he gets hit by PK fire he can't get out because he is so heavy and is easily beat.
His down attack on the ground is good (not Smash attack, I'm not talking about the Yo-Yo) because if yo hit A fast enough you can deal multiple hits easily, again a Heavy character is best example for this because they can't get out easily.
Now PK thunder is great because Ness's is fast, and i don't know f you nitced but you know the little line that follows the PK thunder ? Well you can hit people with it and they stay airorne for about 1.5 seconds
His PK Magnet is good because like the shine it can slow down his fall little, and both Lucas and Ness's PK Magnet have a speciality. If you're close to someone and you do Lucas's PK Magnet it will shock people, Ness's however does no such thing, it instead pushes them.
I luv Ness especially cause he is very versatile, and can take on almost everything that comes onto him.