As the title says.
Make a short list of your favorite (dungeon + raid) encounter(s) for each exapansion included vanilla. Comment why if you'd like to.
My list will be something like this.
Vanilla
Dungeon: Darkmaster gandling. I like the concept where you are being teleported awey from your group and then you have to clear your path towards your group.
Raid: Hakkar (I didn't raid that much during this period as I were not that active, but I got to experience MC + ZG with pugs.)
TBC
D: Pandemonius
R: Mother Shahraz, Illidan, Kael'thas(pre-nerf), Kalecgos and Archimonde
Wotlk
D: The Lich King event in Halls of Reflection
R: Mimiron, LK, Thorim
Cata
D: General Husam, Lady Naz'jar
R: Magmaw, Council and Cho'gall
I've haven't been in touch with all raid bosses/encounters, but I based my opinion on what I've experienced.
Try to limit the encounters by 5 each Dungeon and Raid for each Wow period. (vanilla, tbc etc.)