Hmm for the entrance bosses , I will say Flame Leviathan (4 towers) because of the originality and the esthetic aspect of the fight (color balls falling from the skies).
Lord Marrowgar has an honorable mention : he was not hard but his skin was pretty impressive for a first boss
.
Middle boss : Algalon and by far , the space , the lore , the voice , the fight . He is my favourite boss ever
. Too bad Blizzard decided to ruin him with those bugs >_>.
End bosses : Lich King is the first final boss I defeated with a guild , that was so epic. Yogg and Illidan let me a good impression too and I can't wait to fight Lei Shen.