A deeply rich story, funny dialogue, and great puzzles that will truly make your brain think. The story is very rich and spans across several different eras of Aperture history, going as far back as the 50s. The dialogue is funny and some of the lines are the most memorable in all of gaming (like the Cave Johnson lemon rant). And last but not least, the puzzles are great. They start off pretty simple, but as you progress further in the story, they get more and more complicated, especially when you get the repulsion gel and proposion gel. I feel like Portal 2 is the Gold Standard for puzzle games that every game that comes after it will be judged on.
Also, if you don't own Portal 2 yet, now is a fantastic time to get it - it's on sale for $1 on Steam, same with Portal 1. And if you want both games, the bundle containing both games is $1.50. Do not miss out on this offer, it's so worth it.
My favorite moment of Portal 2 is easily Cave Johnson's lemon rant. Easily one of the best quotes in all of video game history.
"All right, I've been thinking. When life gives you lemons? Don't make lemonade. Make life take the lemons back! Get mad! I don't want your damn lemons! What am I supposed to do with these? Demand to see life's manager! Make life rue the day! It thought it could give Cave Johnson lemons! Do you know who I am? I'm the man who's going to burn your house down! With the lemons! I'm going to get my engineers to invent a combustible lemon that burns your house down! [COUGHING]"