Helps him the most?
Well... to answer that question, you have to take details into consideration.
Consider the fact that even though Fox/Falco lasers are really good, Fox and Falco are much faster and better at using them, and also have Shines whereas Kirby does not. And even Fox lasers can knock Kirby's hat off.
Marth and Roy Hats are next to useless, for obvious reasons.
Ice Climbers usually have twice the firepower as Kirby's Ice Shot. If Nana isn't around, then he's about on par with Popo's neutral B, but not like it helps much.
Let's face it, Yoshi's ability is worse than Kirby without any hats.
The DK Hat packs a punch, but the range on it is laughable compared to DK's. Remember not to use it off the stage as it takes all jumps away.
Needle Storm is useful, though Kirby can't move as quickly as Sheik to take advantage of it. But Nayru's love is pretty limited.
Toad copied from Peach can be used during recovery to stall in the air, and it does have the potential to counter-attack. But... yeah...
Falcon/Ganondorf Hats can be used for recovery too since any horizontal distance is good for a Kirby coming back from up high. Landing one of these on somone is too good.
Mario/Doc fireballs/pills can be useful. Not as easy to spam as with the original character, especially considering Capes/Sheets can be used in retaliation.
Boozer Breath is fun and funny.
Mewtwo/Samus blasts: these are some of the best to copy. Not only do these characters not have anything inherent to reflect them (Confuse doesn't count), they give Kirby much needed KO potential. They're also long-reaching, also a plus. Additionally, Mewtwo and Samus aren't too hard to catch with Swallow. Blasting with a reverse-B can give Kirby horizontal recovery, too.
Ness - PK Flash is pretty bad as it is. It's even worse when Ness can just watch it come at him and then hold a PK Magnet to recover damage. He can even do this most of the time while recovering back to the stage.
Mr. G&W wins for coolest hat, but his Sausages aren't all that great. It's worth noting that even G&W can use his bucket against Kirby Sausages.
Link/Y.Link - Bow isn't bad... not great.
Pikachu/Pichu - Pika's is very handy to have if only to make sure he doesn't camp you with projectiles. Pichu's would be at least as good to have, but Kirby damages himself the same as Pichu.
Jigglypuff's Rollout has it's uses, unconvential as they may be.
Luigi's fireballs really don't help much, regardless of the fact that he doesn't have anything to bounce them back at you. They're pretty slow and leave you open, which makes it harder to use than if Luigi were to use them against Kirby.
So, in short: Charge Beam and Shadowball are top tier. Lasers and Needle Storm are also pretty good. Mr. G&W hat and Bowser hat are too good.
Note, the above is a perspective of Copy used in Singles. There are other applications used in Doubles.
