Solo run of Wailing Caverns located in The Barrens. Good for levels 17-20, this is one of the most confusing instances to find your way around in. You will get lost the first few times through here. Summoning the final boss also requires a lot of running back and forth around the instance. Four Serpent Lords must be killed before you can start an escort quest which ends with defeating Mutanus the Devourer, a giant murloc. There are a lot of good drops for level 19 leather wearers in here.
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.