Holy Priest, disclaimer that of course skill rules over all, these are just my experiences in the LFG struggle. Also worth noting is that all of this is from low level mythic+'s (2-3) and lower dungeon content. When I run higher mythic +'s I do them with a beast paladin tank in my guild nearly exclusively.

Least favorite are DH's and Monk tanks. I know there are exceptional monk tanks, I just never really seem to see them. Most of the monk tanks I get grouped with (when I do, they are relatively rare) are both insane and shitty. They pull more than they can handle, use their crazy mobility to run away from me and around a corner, and make me panic feather over to them and bust cooldowns just to keep them up on a pull that is cake with a warrior, paladin or bear tank. They also are usually the ones I get that don't 'finish' a pull, and they leave me with the tricky choice of staying with the dps to make sure they don't die to the 25% health mob pack the tank left, or to chase the insane tank who just pulled two new groups. I am all for speedy runs, but calm down dude.

Most favorite are warrior tanks and bear tanks. Warrior tanks feel comfortable to me, I just like grouping with them. Bears are great because they just don't take damage all that often and when they do my big BOOM heal is usually off cooldown again. It's handy.