Sproul has taught me most of what I know about reformed theology, so I'd have to choose him. But I also enjoy Piper occasionally. I love Sinclair Ferguson and wondered why he wasn't on the list, Tim! Haha! I've never heard Tim Keller, but I'd like to sometime.
Christ Church ARP, NC
"Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us for adoption as sons through Jesus Christ, according to the purpose of his will, to the praise of his glorious grace, with which he has blessed us in the Beloved." Eph. 1:3-6 ESV