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Any of his Let's Plays that come to a positive denouement. In particular American Mcgees Alice. You just can't help but be happy for the guy who is so obviously happy himself to have succeeded. He's a definite example of the Good Feels Good philosophy in video gaming (which, given the tendency of people to be jerks when playing moral choice based games, is very refreshing.)
There's just something about that quiet "whoa" at the end of Portal 2 when the turrets are singing for Chell, intended or not, that tugs the heart strings. Then again he's basically saying what we were all thinking.