Whooo, here's a fun thread.
Firstly, I absolutely hate the Styles Clash, though the springboard 450 and Spiral (not spinal) Tap are both great moves.
Christain's Unprettier/Killswitch is pretty good, though I prefer the original name, the Impaler from the Brood days.
The Black Hole Slam is good because it can just happen out of nowhere and looks like it will really knock the wind out of you.
I like the Cradle Shock alot, though I don't know about the whole leg-crossing bit.
The Shellshock is one of my personal favorites, though Shelley doesn't get to use it all that often in TNA. (You also don't get to use finishers when you're constantly jobbing, thanks TNA!)
The Canadian Destroyer will always be at the top of the most impressive list just because the first time anyone sees it, it causes the jaw to drop, even if it's a little less than realistic. Though it is much more believable whenever Petey would hit it 'on the run.'
Though it's a double-team, I'll have to say my favorite TNA finisher is Made in Detroit. I marked out hardcore for it the first time I saw it in a match in Japan, and even moreso when I saw them use it in TNA. I'm a huge fan of tag matches and double-team moves (not to mention MMG in general) and that one takes the cake for me.