Anything GLaDOS says:
-After the test is done, you will be missed.
-STOP!! What are you doing!! I I iiieeeeeeerrrrrwwe are pleased that you survived the part where I pretended to murder you.
-That part you destoyed isn't important to me. It made shoes for orphans; nice job destroying it, hero.
Anything said in TF2:
-What's that sandvich? Kill them all? GOOD IDEA! BWA HA HA HA HA!! -Heavy
-Nice headset, can I see it? "This is Scout. Rainbows make me cry. Over." -Spy
-If God had wanted you to live, he would have not created me! -Soldier
-Eins, zwei, dr-ugh... I don't think we brought enough body bags. -Medic
Some dialogue from HL2 (and episodes):
-Dear God! Poor Laszlo! The finest mind of his generation, come to such an end!
-I used to kill Hunters with my bare hands!
Plus sometimes when rebels are talking with each other they say stuff like:
Guy1: Sometimes, I dream about... cheese.
Guy2: Well what am I supposed to do about that?!
Valve really knows how to write some hilarious dialogue.