Greenbacks or Vintage 30s are both good options -both classic sounds, but different.
Listen to clips of a few different speakers (Blue Bull Dog, Greenback, Vintage 30, G12H, Jensen C12N)
here at Proguitar Demo and get an idea of what you are looking for. They are used on a Dr. Z Maz 28 amp, which isn't a Marshall (but has a bit of a Marshally character), but it gives you a good idea of the differences between them.