Cycling. I'm on the bike 8-12 hours a week. Split between the road, my indoor trainer, and the trails. Hill repeat intervals on the road bike are easily the most intense cardio workout I've ever done.
My passion is XC mountain bike racing so the 8-12 hours spent in the saddle is basically my offseason training for the upcoming race season. The side effect to all this riding is getting ripped to shreds.