Firstly, my choices:
1. TLC 1 - Summerslam 2000
The original, took everything from WM and upped the crazy ante even more. Ladder matches were never the same after this.
2. Shawn Michaels vs. Triple H, Non-Sanctioned Match - Summerslam 2002
HBK returns to the ring after four years and immediately has a five star classic. And since it's one of his buddies, Triple H finally gets motivated and has his first good match since return from his own injury.
3. John Cena vs. Randy Orton - Summerslam 2007
During John Cena's, "I can have a great match with anyone" phase, some fantastic near falls and a chance for Orton to finally step up and return to the main event scene.
4. Jeff Hardy vs. CM Punk, TLC - Summerslam 2009
The conclusion to one of my all time favourite feuds, some righteous bums taken by both guys and some great yet simple storytelling. I give Hardy all the credit in the world, he did his best to get Punk over before he left.
5. Stone Cold vs. Kurt Angle - Summerslam 2001
Would have been higher if not for the shitty finish.
And just comment on some of the other choices made by people:
1. Bret Hart vs. Owen Hart - Steel Cage Match - Summerslam 1994
Sorry, but I seem to be the only human alive who thinks this match is overrated.
4. Eddie Guerrero Vs Rey Mysterio - Ladder match for "Dominic's Custody" - Summerslam 2005 (btw, it was a horrible storyline)
I really want to like this match as it has two of my all time favourites in it. But it's full of botches, the story overshadows everything, and it's tough to watch scarily big Eddie nowadays.
2. John Morrison vs CM Punk - Summerslam 2007
Surprised to see this one. Even Punk himself admits that the PPV series between him and Morrison was trash, and the ECW title match on TV was the only good one. Each to his own though.