Considering I can't ever stick with a strength training plan or cross training regimen for longer than, oh, a week, my favorite workout is running.
A nice, long run is far and away my favorite way of working out...one where my thoughts turn peaceful and my stress melts away. One where my body works hard enough to shake out the cobwebs. One where my body is tired but my mind is rejuvenated.
However, if I have to pick a "workout," I'll go with negative splits on the treadmill. I run in the hot garage before school two or three days a week once school starts back up. Wednesdays are for speed work. One of the most satisfying workouts out there is to kick up the speed every half mile until the end. Sometimes I give myself a recovery in between...but most of the time I don't. I think this workout has been part of the reason why I can finish runs strong, even if I struggle for the first two miles. My mom and sister joke that I'm like Shorty, an old Shetland pony we had once. That horse would see the barn and head for home, even if he'd only been out for a little while. Like me. When I know the run is almost done, I pick up the pace and head for home.