I'm going to disagree with Snake's being second cheapest, as it's fairly simple to time airdodges to avoid successive grenade shots. Just being in constant random motion goes a long way towards dodging it ime.
Now for my own:
Cheapest:
Sonic: No explanation necessary. Anyone who doesn't kill with this shouldn't be playing Smash.
Pikachu: Similar to Sonic, but less knockback and harder to control by several magnitudes. Still lasts far too long for its own good. Also does obscene amounts of damage.
Fox/Falco/Wolf: Grouped for obvious reasons. Lasts too <censored> long, and can KO you in any of 5 ways last time I counted, all of which are easy to do. Fox and Falco's last long enough to KO you twice -- once normally, and again by carrying you offscreen when you fall from your recovery platform (which doesn't last long enough to keep you safe).
Worst:
Ice Climbers: Only affects center stage, doesn't work if you're in the air or hanging onto the ledge, doesn't work if you're far enough away from the Iceberg, and the Ice Climbers can't even climb the blasted thing. If you're on the opposite side of the stage as your target, you can actually be unable to reach them, while they're free to stall in the air/on the ledge until the thing melts.
Zero Suit Samus: Does piddly damage for a Final Smash, has almost zero range, and forces a transformation. I'm sure ZSS was a balancing part of the Zero Laser as a whole, but did they have to take it this far?
Peach: If she catches you off-guard with it somehow, you're wide open for a fair or whatever. But there's actually a fairly large window between the time she activates the Final Smash and the first frame of sleep, long enough to get airborne and avoid it altogether.