Making a quick list for me;
DK: T19 is literally perfect, T21 is great too.
DH: T19 is the only great set they got IMO. Parts of T20 are nice but I don't like the robes.
Druid: T19 is OK only if you're NE. T20 is great, T21 also really good.
Mage: T20 is great, other two not so much but hey.
Monk: T20 is one of my favorite sets in the game actually.
Hunter: T19 is one of their very few actually good sets. Sadly it mostly looks good on elves and humans.
Rogue: T19 is perfect ninja, T21 perfect pirate.
Paladin: T19 is good, but T21 is the real winner.
Shaman: T19 and T21 are equally amazing.
Warlock: as I said, all tiers are amazing. Locks do tend to have the best sets but hot damn.
Warrior: I love T19, T20 is also good.
I can't think of any other expansion where I can praise the sets so much. T17 Mythic ones are also darn great but still not quite as good, but then you have HFC which has ugly shit all over. Mists had the overall great CM sets but the actually raid tier ones are bad for the most part.