A friend - because all healers are viable and it’s more about socializing.
Druid - Simply because of their easy playstyle then also their utility, like vortex etc.
Monk - Mostly because of utility, but their also very bursty healers.
Paladin - They try very hard and it’s rewarding. Utility ain’t bad either.
Shaman - Their usually not bad, however using their full toolkit is hard in a PuG.
Priest - They were FotM quite recently and the Disc nerf combined with Holy being a bit “meh” for higher keys makes the class a wild card unfortunately. A good priest will still function however.
Tanks-
A friend, since just as healers their all viable for the content i pursue aka +10-12’s.
Warrior - Most warriors are really good tanks since they persisted trought the draught and came out stronger then ever post buffs.
Paladin - They just feel safe.
Deathknight - Their overall still the superior choice. But since their FotM many players roll them and the risk of getting someone horrible is high.
Demon Hunter - I’d say their like 2nd best overall, however same reason as DK’s their a wild card.
Monks - Their really good, but they are also fairly rare.
Druids - Sometimes they just can’t do much unfortunately.
DPS-
Friends again.
Rogue - That cloaks just to handy.
Demon Hunter - Seriously Overpowered in mythic+, selfsustain, stuns, +magic damage taken debuffs, decent AoE, offensive dispell etc. Their like a cookie cutter M+.
Mage - Since i love to have the food really, if people have it they might acctually use it between pulls and stop wasting my valuable 5 seconds grace period drinking.
Elemental Shaman - Well I sort of like them, being able to dispel curses, decent AoE, Slow, stun, earth elemental savior capabilities, offensive dispells, ranged interupt on short cooldown. They also have a Zerker buff.
I whould say most DPS end up beyond this point. It mostly depends on what sort of classes you already have in group. For example if you don’t have a battleres mechanic at this point, Druid, DK or Warlock will fit here.