Oh, how could I forget my baby Mk II!? It's just so adorable, and the Dreissen is also pretty cool even if I always preferred Zaku/Gelgoog-inspired designs before Dom-inspired designs. Geara Dogas are also very cool.
A very underappreciated MS would probably be the Rig Shokew and the Imperial Rig Shokew from Victory. They just look so awesome!
http://www.mahq.net/mecha/gundam/v/zm-s22g.htm