Well, Lucario's and Ike's look the coolest but uhhh...if you can use it well, Fox/Falco/Wolf's final smashes are beast in free for alls. I've gotten four kills with one final smash with Wolf. Luigi's is the best one on one sort of. If it sleeps them...you can instantly KO them and when they spawn back they instantly get sleeped again and you can kill them again. Except for a few, most final smashes can be dodged completely such as Diddy's and Yoshi's. Lucas and Ness have very good ones. It can be very tough to dodge if you don't anticipate it correctly. I have to say either Yoshi's or Ice Climber's final smashes are the worst. They almost never kill and they are incredibly easy to get around. During Ice Climber's Final Smash, it's hard to get to the other side where your enemy will most likely be.
Okay...I haven't clearly said which ones are the best and worst but uhh...yeah...Fox's is the best and Yoshi's is the worst.

Mr. Game & Watch = best. (IMO)

DR. OCTOGONAPUS!!

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