Marth and Ike's Counters are insane if you know how to use them and can be great Boss Killers on harder difficulties (Lucario's Counter is good too, although I'm not a fan of Peach's). Both Marth and Ike have deadly Final Smashes and Up-Specials that can cause good knockback and damage at medium damage. I'm not that fond of Link's moves though. I can see how he's good but I'm just not that great with him.
The great green thunder Luigi though...whole different story. With his Up-Special, Side-Special and even Down-Special all being good for recovery combined with his great aerials and floaty-ness he's grat in the air. On the ground his Up-Special is to be feared if you can score a direct hit as around 50 or 60% damage and above it's almost a certain killer if nothing's above you and you're on a medium or small stage. His fireballs also defy gravity and knowing when to use his side-special, the green missile, at the right time can be a great way to add to the confusion, stop items being thrown at you, dodge, recover and even KO.
His Negative Zone Final Smash has the potential for you to possibly KO all your opponents at least once (although sometimes you won't be able to KO anyone) and grab some key items for when it runs out.