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.

Combat arms ranks up fastest. Infantry the fastest of all.

91B points for E5 are prettt decent, but E6 and will take some time.

How about this. You worry about being worth the rank you are, being the best E2 you can be, and then maybe you will get promoted.

Any Combat Arms PMOS is regularly first up the tree (no pun intended for all us apes).
Everything else is slower unless you're the top shelf person.

EOD. Saw plenty of brothers make E-7 and E-8 in 10 years. Even had one guy make E-9 in 13 years. Never heard of that from any other MOS. If you don’t make E-6 in 4-5 years then you have problems and should get out!

Back in the 70's, I went into the army as E-3. In 6 years U was an E-6 and when I left at 8 years I had been an E-6 with 2 years in grade. I was in Air Defense Artillery. I'm quite sure things have changed. I went before the E-5 board as an acting jack even though I was an E-4 Specialist.

I made E-5 in 2 years and 7 months. 11Bravo. Long ago, 80-83.