The most intense for me is tabata sprint intervals.
Get to an oval, sprint 20 seconds (max intensity), rest 10 seconds; repeat this cycle 7 times. There are many studies indicating this effective. However, I like to take the session longer. After each 8 cycles, I have a 2 minute rest, and do it again another 2 sets of 8. Note it took me quite a while before I could do this. First time I was bugged and wanted to vomit before even completing the 4 minute exercise circuit.
The tabata protocol could be done with front squats, lunges, power cleans etc, and other cardio machines (e.g., bike, elliptical, strider), but sprinting on oval is more intense.