Voted GLaDOS, though honestly, my favorite is a three way tie between her, Chell, and Doug Rattmann.
All three are insane survivors- GLaDOS was torn apart and killed, and she's right back to the bitchy scientist the second she's switched back on. Chell... the thought of someone being able to go through the Enrichment Center and not die is absolutely astounding, and that alone proves Chell would be a strong character, if she steps out of the full protagonist spotlight and is allowed more of a personality. Doug... the man slowly slips deeper and deeper into madness, but he survived GLaDOS' takeover, he is responsible for Chell even being able to run the test, and he managed to live in the Center. Actually live- Chell was in stasis or testing most of the time, and everything else is a robot. He's the only person who actually has to live there.
However, every character in Portal/Portal 2 was fantastic, and I salute the writers for putting together such a great story.