Have to echo shadow priest. It's a spec that you don't see performing well very often for several reasons: first is that you can't really cast much of anything while moving, so you have to learn how to minimize your movement while still properly dealing with mechanics. I realize that there are other classes/specs that suffer from this same issue, but it's especially important for spriests because they simply can't do competitive DPS if they don't master the art of minimizing movement.

Another reason is because the skill cap is rather high, so the majority of spriests only use like 4 abilities regularly, and a rotation like Mind Blast -> spam Mind Spike 3 times -> Repeat until 3 orbs, Devouring Plague -> Insanity -> start over. This is the most simple and lazy/noob-friendly rotation an spriest can do, but the DPS is really lackluster, so you'll often see spriests near the bottom of the damage meters.

A very good spriest can actually top damage meters, and uses a fuckton more than 4 abilities. There's a high enough learning curve with the spec that it takes a much longer than average amount of time to get to this point, unfortunately most people get frustrated with the bad DPS, disregard it and give up before it finally clicks. You just don't see too many spriests doing well, so it's always a rare and impressive sight when you do see one.

Disclaimer: I do main an spriest, but I'm not a super amazing player. I am usually between #1 and #4 on the meters, but my ilvl is also a little lower than most of the group. I generally parse around 70th percentile for my ilvl bracket for most fights, but 50th or sometimes embarrassingly lower on one or two in HFC (this is most likely because I only play Clarity of Power, and Auspicious Spirits is strictly better on those encounters). I've personally had the pleasure of raiding with 2 or 3 spriests that just blew me out of the water though, and it's really a joy in those cases to see the full potential that's there.