A guy who saved NBC Late Night's sorry ass twice in one decade. First time was in 2000 by resurrecting Weekend Update (with much help from Tina Fey), turning it into a fun and clever guy-girl exchange that worked wonders. Second time was in 2009 after the whole Leno-Conan reshuffling clusterfuck didn't work out, leaving Fallon as NBC's only non-fail comedian (out of four) still standing.
Writes jokes that are smart, relevant, and actually funny (unlike Leno). Also unlike Leno, his comedy sketches actually reflect some effort and creativity (i.e. Remix the Clips) instead of being tired ripoffs or just mooching off user input for humor. If there were justice in the world, Fallon would host the Tonight Show and Leno would get bumped to 4:03 AM on Telemundo.
A pretty good impressionist and parody songwriter, too.
Conan at his peak was still the best, but as replacements go, you gotta admit this Jimmy Fallon motherfucker is aiiiiiiite