My hardest workout was 2x 800, 400, 400, 200, 200 @ go pace (all out)with full recovery (1 lap walk...hardly full recovery for me)between repeats. When one of my teammates asked our coach what go pace was he said "go pace..go as goddamn fast as you can." That has to be one of the best coaching cues I've ever heard. A few of us were puking after that workout.
A little of the topic, but my most boring workout was 25x400m...1 lap in 80 followed by 1 lap in 100 sec non stop. My faster teammates went 70sec/90sec.