For me personally, you have to go back to UFC Fight Night 2. Josh Koscheck vs. Drew Fickett. Koscheck is totally dominating Fickett for 14minutes and 20 seconds. We're talking there is no question Koscheck is going to win this fight. Even Cecil Peoples couldn't screw this up. Then with about 40 seconds left, the ref stands the fighters up due to inactivity. Fickett catches Koscheck and stuns him, then sinks in a deep RNC and puts Koscheck to sleep. No tap, straight to dreamland at the 4:38 mark in the 3rd round. To me this is my all time favorite finish and comeback victory because Kos was unbeaten up to that point and afterwards learned to finally be humble.
Another one is Scott Smith scoring a KO of Pete Sell at the TUF 4 Finale. Sell had Smith rocked and down on his knees when he charges in and gets nailed by a bodyshot that ends the fight. To this day, two of my favorites.
And one final is Gegard Mousasi's KO of Ronaldo "Jacare" Souza at DREAM 6 in the Middleweight GP Finals when he KO's Souza with an Upkick to the jaw. Beautiful.