The most underrated exercises are anything that is NOT in FLEX or Musclemag.
I honestly believe that popular publications that people read focus on so much crap and nonsense that most trainees think Kettlebells, Bent Presses, Olympic Lifts, Front Squats, Overhead Presses from a standing position, and a lot of other good stuff is useless. If MILO were as available as the rags, then all of the stuff mentioned wouldn't be underrated, it would be widely practiced, and instead of having 15 year old sticks who weigh 140lbs walking around in wife beaters acting like they actually have lats, we would have 15 year olds who actually deserve to wear a wife beater and who have a body that is actually impressive.
That being said, Overhead Squats, 1 Leg Squats, and Standing Overheads I feel are very underrated.