Here is my opinion. Drop a comment if you disagree.
Top Tier
Diddy Kong: Everyone dies. Leaves destruction and tasty snacks in its wake.
Sonic: Everyone dies. Even if this skill ends with you offscreen, Sonic's awesome recovery means you'll probably be able to make it back to the stage. People who spam counter attacks, super armor, and dodges might be able to live a couple seconds longer than the ones who get instantly annihilated.
Ganondorf: Everyone near you or in front of you dies.
Link: Free kill to anything in a line.
Zelda/Sheik: See above.
Toon Link: Same as Link (less range, more knockback, shorter animation).
Fox: Assuming the person knows how to drive a Landmaster, this always deals the awesome.
Falco: Flying to the top of the screen is just too hilarious not to do.
Wolf: Don't drive off the edge. Your projectiles are much stronger, but you have less recovery and maneuverability.
Samus: Mega Death Laser obliterates everyone in front of you. It's easy to catch 2 or 3 people with this. Kills at ridiculously low percentages.
Pikachu: This is harder to control than Super Sonic. Assuming you can aim it, this skill will probably kill everyone who isn't camping on the respawn platform. This skill is only high tier if you know how to move this ball of death. Unlike Super Sonic, people won't have any luck dodging this.
Mid Tier
Mario: Mario is always setting the standards for mid tier. His smash is near-death in a line. Not as much power as other smashes.
Luigi: Keep airborne and you can get out of this. Luigi can't act for the first half of it, giving you the chance to get away. Getting away is much harder than it seems.
Yoshi: The fireballs only go horizontal, so this one isn't too hard to evade. Said fireballs are ridiculously powerful.
Wario: The turbo-hog instantly kills everyone. A good Wario won't die while using it.
Zero Suit Samus: Free kill, but you have to really close distance with someone. That shouldn't be a problem with Zamus.
Kirby: You can dodge this with counter attacks (and I think air dodging and edgehogging too). This brews items if you're playing with them on.
Metaknight: Free kill. This one is a bit harder to land. MK might have the orb smashed from him if people attack from from a distance, or from above/below.
Pokemon Trainer: Death to anyone in a line. People can still move during this (even though they can't be seen), so it's particularly annoying to land.
Marth: Free kill to whatever's in front of you. It's not high tier for two reasons. First, you can't easily hit multiple people with it. Second, if you miss, you'll be shot horizontally, possibly to your doom.
Ike: Massive damage to everyone who gets caught in it. Doesn't kill unless the opponents have some damage on them already (heavyweights will need to be really battered).
Ness/Lucas: Starstorm causes projectiles to randomly fall. Characters who get hit twice in a row usually get KO'd. This skill is bad news to anyone already significantly damaged. The randomness of the skill could mean that some areas barely get hit at all. Weak in 1v1.
Snake: It's show time. It's possible to evade this with some hectic scrambling around and spamming dodges. If you have skill enough to hit people in the air (or vertically off screen), then you've got some free kills (or at least easy damage).
Captain Falcon: Harder to hit with, easier to avoid. This kills at low percentages.
Low Tier
Pikmin & Olimar: Deals negligible damage (assuming you evade the explosion). Hold on to the edge during this smash and stay away from the center of the stage to easily avoid the kill explosion (you might have to jump away from the stage during the explosion).
Peach: Peach can easily snag the smash ball with her ridiculous air control. Her smash gives her a free smash attack on everyone. In larger games (or on larger maps), this smash can be better for your enemies if they manage to snag the food that spawns.
Donkey Kong: Deals damage in a huge AoE (usually the center of the map). Since he moves up with the screen, he's not likely to die in scrolling stages. Not as good for KOing as other FSs.
R.O.B.: The face laser. This puts insane pressure on your opponents. It doesn't actually help much in scoring KOs.
Bowser: You still have to catch people and kill them, and you can still accumulate damage. Useless on very large maps (like Hyrule Temple, blah).
King Dedede: Avoid the spikes and you should survive. Hanging around the edge of the map is a practical way to avoid this.
Ice Climbers: Again, depends on the opponents and the map. Rather useless against players who can hold their ground.
Pit: It's not hard to dodge these buggers. They attack at set intervals and make sounds when they do, so everyone will be focusing on dodging. This could open up a window for Pit to land some attacks while people are busy avoiding the swarm.
Lucario: Move to the left/right of him, or jump over him. Some characters can even cross the beam (for example, using PK Thunder 2 allows this).
Jigglypuff: Grows huge. Anyone touching Jigglypuff when she screams gets shot off the map. This is probably the easiest final smash to dodge.
Game & Watch: Transforms in to a malevolent octopus that can only jump once. It can't chase after other characters, so it feels something like the hammer.
And here's one of the top tier final smashes in action!
Hell we got turbo 
