Tim Keller is my favorite preacher, pretty much no doubt about it now. I've never heard anyone integrate apologetics, relevant insight into Scripture, and the supremacy of Christ so seamlessly and effectively. I'd encourage you to check out some of his stuff this summer, and it certainly wouldn't hurt ot recommend him to unbelieving friend or skeptic.
Here's something Keller said about preaching:
Where Did All These Calvinists Come From?
53 minutes ago