1) Halls of Reflection (I like gauntlet styled dungeons and this instance was also difficult in the right way, interesting tactics like stacking on the corner before waves came for example)
2) Magister's Terrace (Most difficult 5-man in TBC and also probably the instance I've been to the least, it's great Blizzard allows players to enter this at lvl 68 instead of lvl 70 in the next patc (PTR))
3) Culling of Stratholme (Again, a good "gauntlet" styled dungeon, there really should be more like these, fighting alongside Arthas felt Epic)
4) Halls of Origination (Lots of bosses with fun mechanics)
5) Shadow Labyrinth (Fun bosses and the last boss Murmur felt like a raid boss back in TBC, I remember an Orc Warrior tank charging in to fight him which looked awesome)
(Posted this again with descriptions because Spinoff: top 5 worst 5 mans.