One of the best I've heard so far is his Who is Jesus? talk in which he says,
The magnitude of Jesus' claim [that he is God] is such that you'd better not just doubt that he is God...you'd better know it for sure. If there's a chance that it's true and you miss it...you're ruined.There is a useful compendeum of Tim Keller talks and articles here. I've particularly enjoyed his talks on Redeemer's vision.