Chins and pullups. There is nothing like getting to the end of the set and feeling the strenght just ooze out of your body. And the burn, oh the burn. Biceps are on fire, lats feel wasted, the grip is starting to let go but I have to get two more reps out without kicking my legs, not matter HOW much I want to. That's when concentration wins over fatigue, when those last few reps feel like they take longer than the entire workout.