The problem with this is that the characters in the game don't disintegrate / burn / crush / deafen (DK's) when they get hit, like you would.
From a realistic point of view - Captain Falcon's would be one of the worst as far as gruesome goes, but you wouldn't experience all that much pain. Same with the giant lasers (Mega Man's, Palutena's, Samus's - by the way, Palutena's and Mega Man's wouldn't actually be black holes - considering the opponents don't take damage). Seriously - any laser that large is going to be like going into the sun - you disintegrate instantly.
Pikachu's FS really scares me, considering you would probably experience being electrocuted for a good few seconds before your heart stops (and then you burn and die). Kirby's from Brawl would also be one of the worst.
As far as the sword slashes go - Ike's seems like the worst to me. Whereas with Link's where you're trapped and so he's just slashing at you, Ike is literally hitting you hard enough to keep you up in the air. Plus, if you survived, you're going straight down to a fiery doom.
And for the one's that you might not expect to be painful - Luigi's Poltergust. Think about the space that you're getting stuffed into, probably breaking your neck, not dying instantly but rather experiencing horrible pain throughout your body until your body shuts down because your arteries have been pierced and you're no longer receiving air to your heart and brain. ROB's from Brawl would also be painful - because you're getting burned / disintegrated by lasers over a wide range of your body without anything vital immediately getting destroyed.
