Some comments on what other people said first: R-Truth's lie detector looks pretty weak, but it has some flash, so I won't call it the worst. Yoshi Tatsu's kick to the head isn't that impressive since plenty of guys (CM Punk, Morrison, even Shelton to an extent) give kicks to the head and doesn't look flashly like sweet chin music or trouble in paradise. The zig zag has a setup and is usually timed well, and there's decent impact, so I don't think it's that weak at all. The KO punch shouldn't be in this discussion, in my opinion, unless you're commenting on how it's hard to sell a KO punch when the guy can't actually hit you. Because yes, Show could technically hit it whenever he wants, but he doesn't because he has to "soften up" his opponent first, and I buy that. I also buy that his punch is worse than anyone elses, since he's the biggest in the company. Edge's spear only looks weak to me when he does it to someone huge (like the Big Show last night). Even with a head of steam, he couldn't take Show off of his feet like that realistically. The future shock looks awesome, I understand that it doesn't always look like the guy's head hits, but that's a problem with who is selling the move, not delivering it. Paydirt would look awesome if someone knew how to freaking take the move. Ziggler f'ed it up last night on Smackdown. The generic name of that move is called a "Complete Shot" and you're supposed to land on your face, not your back, ugh.
Moves that I think are as weak or even weaker than the above mentioned ones: The attitude adjustment (he is a power wrestler, yet he doesn't really slam his opponent with this, he just throws them. A superplex would be higher off the mat with more impact, THAT would be a better finisher in theory), the 619 (if you count it as his finisher, that is. If you hate the Lie Detector, you have to hate this one, because even though this has a setup, all he is doing is delivering a flashy kick to the face instead of a flashy elbow to the face. Although lately, guys have been reversing Rey's springboard moves after being hit with the 619, so I guess Mysterio is allowing it to look like the 619 by itself is not powerful at all, which is admirable), Shad's STO (he swings his leg because it's supposed to look like he's tripping his opponent, but he never even touches the opponent's leg! This just looks like a weak takedown).
And because I feel like saying it, the move that should be a finisher but isn't: Mike Knox's "Flying Bear" (I hate that name) running cross-body. Without ranting about how I think Knox is grossly underused, this move has to make everyone cringe when it's delivered. This move looks absolutely disgusting and could easily be a finisher. Since he hasn't won a match in months though, I guess it's good it's not his finisher so I get to see it every week. His finisher is a swinging complete shot that doesn't have a name (to my knowledge, I've been calling it the Knoxout) if anyone is trying to remember what it was.