Superman Punch leg kick combo on Serra, as well as the early takedown he got in round 2 of that fight.
Surviving the Condit Head Kick, to come back and slam him.
Fairly Squarely.
Too many to list. 
One of my fav's was the koscheck fight. Big wrestler everyone said GSP couldn't handle (at the time) and he just sat down on his jab all night long making it a power punch and destroyed kos's orbital.
"Next time, im going to beat him fair&square...orr...sorry...fairly-squarely...Sorry for my Eng-a-lish" -GSP
"I am ah'compeleet-a-ly obsessed with Jahnny Endreeks" -GSP
And my fav,
"You cannot andull my riddum!" -GSP 
UFC 154 Primetime Ending - intense moment when he admits that he's afraid..
http://www.youtube.com/watch?v=jO-Qjqay-Rs
Definitely the end of the 1st round vs Kosheck. GSP looked like the fight had yet to start and Josh was swollen and seemed like the air left his sails.
When GSP kicked Hughes leg right out from under him.
TheDarkPassenger - When GSP kicked Hughes leg right out from under him.Yup. And Hughes looking confused at how he is getting SO whupped. Even looking at the referee hoping it was somehow illegal.
Press conference before GSP v Kos 2, where he tells Koscheck, essentially, that after this fight Kos should retire if he's any kind of a man. That it's going to be a LONG time before he gets another title shot, and a LONG time before he has to hear about him again.
GSP was fucking scary alpha in that presser/
joe canada - Press conference before GSP v Kos 2, where he tells Koscheck, essentially, that after this fight Kos should retire if he's any kind of a man. That it's going to be a LONG time before he gets another title shot, and a LONG time before he has to hear about him again.No disrespect to Kos, but Jesus how GSP was right.
GSP was fucking scary alpha in that presser/
pidgeotto - I'm not impressed by your performance
VU
That was my favorite GSP moment too. It seemed so forced like some of his handlers wanted him to bring some old-school WWF style trash talk and that was his best attempt. The guy just isn't a trash talker.
Hughes GSP II. Matt sits on stool between rounds breathing hard. GSP paces in his corner staring at Hughes the whole time. So so cool.
Making BJ quit on his stool.
tj tapper - Hughes GSP II. Matt sits on stool between rounds breathing hard. GSP paces in his corner staring at Hughes the whole time. So so cool.For a guy that seems very passive, he certainly had some alpha moments
Too many to name but definitely when he won the belt the first time stands out in my mind. Great career, great fighter and an even greater human being. We were very lucky to witness his career unfold.
My personal Favorite
TheDarkPassenger -I like after round 4 of penn2 when he got up and punched the cagetj tapper - Hughes GSP II. Matt sits on stool between rounds breathing hard. GSP paces in his corner staring at Hughes the whole time. So so cool.For a guy that seems very passive, he certainly had some alpha moments
Mine is honestly that he retired. It made me a huge fan. He is the first to do it right and go out on top.
kyle223 - When GSP won the belt for the first time, destroyed Serra in the rematch, and his walkout to UFC 74 when his music starts and he gives the camera a "#1". Fucking incredible.
"I'm not impressed by your performance"
Hughes GSP II. I was thoroughly sick of Hughes at that point and wanted nothing more than to see GSP do exactly what he did in that fight.
Defining moment was coming to the realization as a huge Penn fan from Hawaii, that Penn had exactly zero chance in that rematch. That is the only time I've ever counted Penn completely out in a fight even of I've pegged him as a rightful underdog in a few since. Didn't go to the bar because I just knew Penn was getting steamrolled that night. Fall out from that was unpleasant, but that was when I felt like GSP was going to solidify himself as one of the greats.