I think I've found my favorite leg PT thus far. What exercise will reduce a fresh, well-rested physique into quivering jello and incipient upchucking in the space of five minutes? Sprinting up 30% grades, of course! Charge up hill, walk down hill, repeat until your legs tell you in no uncertain terms to fuck off.
Basking in the narcotic haze of endorphins released afterwards is of course an ancillary bonus.