All of the trash in BWD. All of the trash had its own extra mechanics. Dwarven council for Atramedes trash was especially good.
This tier, it has to be Blade Lord Ta'yak's trash. Never ceases to amuse.
Currently a 14/14 Holy Paladin and 10/14 Discipline Priest in Siege of Orgrimmar
Formerly voidspark - 8/8 Demonology Warlock and Restoration Druid in Dragon Soul
Elegon trash is fun