My favorite event each year is the Yankees-Red Sox series. Love seeing passions run hot among the fans, especially when both teams are in the running for the playoffs.
The best under-the-radar rivalry is Dodgers-Giants. I had no idea how deep that one was until I moved to California... that one goes waaaaaaaaay back, and both sides absolutely detest each other. Fights in the stands, fights in the parking lot, the whole thing. It's every bit as bitter as Yankees-Red Sox without nearly the same hype. Bill Simmons