The first instance on Kalimdor. Located inside Orgrimmar this is the hardest instance to get to for the Alliance, just as The Stockades is the hardest for the Horde. I was killed 3 times while trying to reach this instance. The instance itself is the lowest level instance in the game and one of the shortest. The mobs are around level 13.
Stratholme part 2 of 2. Part two deals with Undead Strat (or UD Strat). This portion is occupied by the undead (obviously). In order to get to Baron Rivendare (the final boss) you will need to get the Key to the City from Magistrate Barthilas.
Stratholme part 1 of 2. Part one deals with Scarlet Strat, the portion of Stratholme occupied by the Scarlet Crusade. Stratholme is lots of fun to do as a paladin as so many abilities do extra damage against undead. In order to enter the Crusader's building you will need the Scarlet Key that drops in Scarlet Monastery Library.
World of Warcraft Instance Run #8. Sunken Temple solo achievement run. The achievement for Sunken Temple is to kill the Shade of Eranikus, but in order to have him spawn you must first kill Jammal'an the Prophet. In order to have Jammal'an spawn you have to kill the six troll mini-bosses. This instance is a nightmare to navigate. I usually end up taking many wrong turns and getting lost. I was lucky on this run and managed to not get lost.