I have not unlocked everyone yet, since I'm slow at playing and not very skilled, but it's about fun anyway. I don't know if I have a top favorite, but I'll just list the ones that appeal to me in a random order; whichever ones pop into my head first.
Bowser's Final Smash is awesome, because he looks even more bad-arse... to me anyway. Even if I am unable to fight effectively.
Meta Knight's Final Smash is awesome too, even though it is hard because of the short range. But when he's all like, "Behold my power," or something like that, and everything goes dark, then a sudden slash.... so cool.
Ike's Final Smash. This shouldn't be surprising. It seems everyone's impressed with his, including me. I think my mouth falls open every time I land a hit with that. I did not know you could hit more than one enemy with it though... this calls for testing...
Link's/Toon Link's Final Smash. I have not unlocked toon link yet, but they have the same one, and I think it's awesome and impressive. Fun to watch...
Zelda's/Shiek's Final Smash. Pretty basic, but cool to me nonetheless. However, I usually miss than hit, but when it does, it shows the slow-motion of their pain... such a wonderful sight.
Marth's Final Smash. It isn't as flashy as most others I've seen, but when it connects, it seems that it's a guaranteed knockout... assuming they don't counter, or dodge, leaving you open for a nasty attack, or if you SD yourself. Good times, good times...
Pokemon Trainer's Final Smash. Can't say much about this one since I just like it. Feel the power of pokemon with... teamwork..? Yeah...
"It's Super Effective!"
Luigi's Final Smash. I am actually surprised I am putting this down. When I found out what he does, I thought it was ridiculously stupid, but when I did it for the first time, I actually laughed out loud. He's got some hidden issues, me thinks...
King DeDeDe's Final Smash. Think whatever you want with the reason I will give, but I actually like the weird music playing, or when he starts chanting, or whatever the heck he's doing... and his little lackies are everywhere. Yay.
Lucas'/Ness' Final Smash. The comets can do heavy damage, and it's awful when used against me, but it's possible to dodge them. I have been able to not get hit by even one, or I get massacred. Depends. I bet it's really chaotic-looking with four people though... *evil grin*
Well, that's all I can think of, since I don't have everyone yet. I have seen some final smashes with online gameplay videos, but it feels that I can't really judge them until I am the one who performs it.