Im thinking like classic -> bc (the heroics in bc). Not to say that the wrath and cata dungeons where bad... or even that WOD dungeons were bad... (btw, fuck panda dungeons). What I liked about the dungeons in classic was the scope. They werent, lets kill 5 bosses and be done with this shit. They were 5 man raids that tested your ability to cope with shit, not to mention they tended to have very amazing immersive environments.
Stratt? Amazing! (Im sad they split this one up).
Maurodon? My favorite fucking instance in the entire game. I might rage quit if they ever "redo" this one the way they've been "redoing" dungeons. grumbles about how they keep ruining great dungeons One of my favorite parts about this was going through it on my Druid and having the quest from the centaur boss that gave you the staff to stick in the stone. Felt this was like a bonding moment for my druid, especially since I was yearning to be a tree under the old talent system (where healing literally meant you where stuck in tree form 100% of the time).
Black Rock Spire? This one is actually a combo of good design, and the negative effects of free will. I've hated this one for years but I was running a friend through it and decided to follow the paths instead of jumping off to the last boss.... and this is an amazing dungeon, especially the lower half which features a very complex put together that really gives some scope to things. Would love to see something like this put back in the game.
I guess I should include that the BC dungeons I enjoy the challenge and scope of are places like underfen and zangramarsh dungeons and the botanica area dungeons. They're challenging for the level and immensely entertaining. Immersive environments, at times even a story. I also love the length, lack of cut scenes, replayability, required strategy and communication. HAHAHA, explaining to a warrior that he needed a gun and even waiting for him to get one from the AH just so he could shoot the blobs in maurodon. lol. warriors = jarheads of wow and the jar is empty at low levels. "wtf healer, why no heal me" Cant heal you when you get globaled by four blobs stupid. "huh?"