Which MOS's have an easier time ranking up and which ones have a harder time?

The muddier you can get, the faster you can get promoted. The more sterile your surroundings, the slower you go.

I notice in your profile that you're in the National Guard. So the real answer here is that it depends on what the main units are in your state. In Minnesota, for example, the 34 ID is very combat arms heavy, and the most spots for promotion (both enlisted and officer) are in combat MOS's of 11, 12, and 19, followed by logistics branches.
If your state's force is more combat support-oriented, then MOS's in the support and sustainment fields are likely to have more positions overall (and so more opportunities to promote up and find new opportunities as a Soldier).
And to echo CSM ZaGara, keep up the work of broadening your experiences and roles in the Army. The longer we stay in one position, the more complacent we get as things become "routine," and complacency kills. Also, we want our people that promote up to have the best range of knowledge and experience.

You want to know the cold hard facts of achieving rank the quickest? You either do Combat Arms or go Preventive Medicine. I can guarantee with 100% accuracy that for PM....you can attain Staff Sergeant (E-6) in 7 years....as long as all your ducks are properly lined up and in the proper row. I went from SPC (E-4) to SFC (E-7) in the PM world in 11 years. But that is USAR. Guard...that is a different ball of wax.