I tested a bunch of spikes in training mode against Bowser hanging from the ledge of Final Destination. Here are the results (note: I didn't test with everyone):
0: Luigi; dtaunt - Guaranteed instant kill
1: Ike's upB; Kills anyone hanging on the ledge instantly; even kills most of the cast when started on the stage (See Edit note at the bottom)
2: Dedede; Super Dedede Jump (falling)- Doesn't kill instantly, but Bowser can't get back.
3: Pikachu; Thunder - Just barely less than Dedede's...Pikachu has to use it just above the lower blast line
4: Ganon; Wizard's Foot - Just barely less than Pikachu's.
5: Falcon; Raptor Boost - Surprised me. Bigger difference from the last three, but Bowser can't get back from this one either.
And for those wondering about these:
Ganon; dair - Strong, but Bowser can get still back to the ledge.
DK; dair and fair- A bit less than Ganon's dair.
Others tested:
Diddy's dair
DK's downB
Dedede's dair
Falcon's utilt
Samus's dair
ZSS's Flip Kick and dair
Sheik's dair
Link's dair
Zelda's dair
Ness's dair
Ike's dair
Mewtwo's dair
Roy's dair
Mario's fair
Charizard's dair
Falco's dair and sideB
Shulk's dair (in normal and Smash arts)
Megaman's dair
Marth's dair
Robin's Elwind
Duck Hunt's dair
Palutena's dair
Kirby's upB
I also tested the top ones on Jigglypuff to be more sure about placings. Turns out Jiggs can survive all of them (except Luigi's dtaunt). Funny... You'd think spikes would pop the balloon Pokémon
Edit: Initially I didn't test Ike's upB spike because I assumed it wasn't very strong. But now upon testing, even when Ike starts the UpB while standing on the stage and then moves forward to suicide, Bowser doesn't even get out of hitstun in time to jump (on Wii U; on 3DS he does, but he still can't get back to the stage). And when Bowser (or anyone else, for that matter) is hanging on the ledge when Ike gets him, it's practically an instant kill.