Solo run of Razorfen Downs by my retribution Paladin. This instance is straight east of Razorfen Kraul in the Barrens and contains slightly higher level mobs, around level 36. This is a good instance for level 33-40 players. To get the achievement you only need to kill Amnennar the Coldbringer and can skip half of the instance.
Solo run of Razorfen Kraul. Found in the south-west corner of the Barrens, containing level 25 mobs. This dungeon can be a little disorienting as there are multiple paths a levels. This is one of my least favourite instances just because of the sounds the mobs make.
Solo run of Blackfathom Deeps. Blackfathom Deeps is the first Alliance biased instance in Kalimdor, located in the north-western corner of Ashenvale. The entrance is located at the end of some long tunnels and is full of naga, satyr, and the Twilight's Hammer cult. The mobs are around level 23.
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.