I stumbled into the Well of Souls yesterday, the Cata-era instance, a dungeon I had almost forgotten about. It was timely (with Legion coming up) to go back and be there as Illidan, Malfurion, and Tyrande faced down Mannoroth. The lore of the dungeon itself was pretty cool, as we were sent back in time to snatch the Dragon Soul and deliver it to the Dragon Shrine in Northrend.
That series of dungeons came out late in Cataclysm. They played pretty fast and were a welcome respite for many after much more challenging instances earlier in the xpac. They weren't very memorable from a game-play perspective, but they were well done as lore and story telling. I think most of us enjoyed them.
I also loved the Zul dungeons that came out in the patch before. Zul Aman and Zul Gurub both felt very challenging at the time. Wipes were common, but the fights were fun and the troll bosses were great villains. I played through those again recently to farm transmog for my troll hunter and was reminded of all the good times had.
Perhaps my all time favorite was the one that scared players so bad that half of them left group during the loading screen: Halls of Reflection. So many epic finishes as we raced to down waves of undead before the Lich King caught us! (and make no mistake - at level 80 he was walking death).
We probably can't judge the instances we're still tired of; I think we'll need to distance ourselves for a couple of years to have that overall perspective.
What are your all-time favorite 5-mans?