STIK FORTRESS 2I don't know about best, but some of the most memorable are from Team Fortress 2.  Quotable as hell and just as hilarious.  I've had entire conversations when all we've said were TF2 quotes.

The characters' voices perfectly suit their personalities and designs, and the delivery of their lines and the acting in general really adds to the light-hearted, humorous atmosphere.

Also, in before hats.