I went with Baseball. I know most people find it boring, but if you watch it, more than likely you'll see some excitement. Baseball is the sport where you see the most rarities happen. An oxymoron I know. If you've never seen a no-hitter or perfect game, it's pretty cool. I was at Justin Verlander's no hitter in 2007 against Milwaukee and I couldn't believe what I was watching live. Fast forward 3 seasons, and again, Im at Comerica Park for Armando Galarraga's almost Perfect Game. Fellas, I've been to more Tiger games than I care to count. That includes the 2005 All Star Game, the 2006 American League Championship Series and the 2006 World Series. I have never seen Comerica Park so electric in my life. Add in things like hitting for the cycle (getting a single, double, triple, and HR in one game), a triple play, a strikeout on a 3-2 count with 2 outs in the 9th inning, a grand slam, etc. and baseball's excitement is there. You just have to know what you're looking for.
To be straight, I think every sport has excitement to it. If I had to pick the most exciting to the fans here in the US, it's Football hands down. The hitting, blocking, running, interceptions, etc. all lend to it's excitement.