Not the highest TSS but the rides I've done where I felt the most thrashed at the end were when I did 40-20s. 40 seconds all-out, 20 seconds easy, 10 times. Rest 10 minutes, then repeat. Effin-ouch.
I can vouch for this. I just did my first set of these last week (I'm only a year back into racing so am slowly building into harder workouts via my coach's training plan). I did 3 sets of 8 40/20. Before the workout I was thinking "this won't be that bad...my 15s on-15s off intervals aren't bad...think I'll add some 1' intervals to this as I'll feel fresh." After the 40s/20s I was basically limping home with acid throughout my dead legs....freaking awesome TSS of 95!!!