Mario: Looks to cover a large amount of range, especially if positioned right. Best single FS so far that doesn't need additional controlling.
Link: As others have said, it looks like it may target only one character, unlike the others, but if it does, they are as good as dead. To early to judge though, as we don't know how it starts up, or how it ends.
Pikachu: Looks to be one of the better ones if you know what you're doing. Looks moderatly powerful though. from that 15 seconds of footage.
Bowser: If you're good with Bowser, this is probably the most deadly of Final Smashes, as it not only compliments Bowser's power, but also makes him invincible. So if you know the character, this is probably the best hands down.
Samus: The laser looks thin in comparison to Mario's FS, but it seems to have more power. If the Samus user isn't good with ZSS, then it's probably one of the good ones. If they are good with her, it could be better.
Kirby: Nothing really known. You can't really comment on it.