Top 3
Operator Thogar. Choo Choo
AU Blackhand
Razageth
Thogar is more of a meme choice but it is my favourite gimmick boss encounter. A more serious answer for that slot would be Daddy D.
Bottom 3
G’huun
Sylvanas
Xavius
G’huun because of how tedious the orb mechanic was. Particularly in things like LFR and Mythic. For opposite reasons lol
Sylvanas because of phase 2. That is all. Too long, should’ve split her encounter into two different ones like Deathwing and N’zoth got.
Xavius because most disappointing mythic boss. Too easy, even a scrub like me got cutting edge Xavius.
Dishonorable mention to Madness of Deathwing.