lol you may need to have a spoiler warning in the title. 

one of my favourite moments is when you are approaching the pit of fire and escape it, this is pretty much the first time you aren't obeying GlaDOS and the game from then on feels like you are trying to escape the labs. 

and yeah I agree, the ending is epic. I will never forget… This was a triumph. I'm making a note here: huge success.

One of my favourite games of all time and is an engaging story everyone should experience. 

P.S. The cake is real =P

The point in the game in the 19th test chamber when you are heading to the fire pit, and the music starts playing, and GLaDOS is all like "goodbye!" and you're like "no way!"
Then the escape section surprised me, and it was quite lengthy (which is a good thing) and of course the 'boss' battle with GLaDOS was funny as hell! 

"Deploying surprise in five, four. Time out for a second. That wasn't supposed to happen. Do you see that thing that fell out of me? What is that? It's not the surprise. I've never seen it before. Never mind. It's a mystery I'll solve later. By myself. Because you'll be dead…"