What is the best MOS to have for potential AGR positions?

Depends on your goals. If you want to make SGM, then 42A probably isn't the route to take. They seem to cap off around E7 in the AGR world. (can anyone confirm or deny?). But I would suspect you will still get 20 good years from it. Also, remember HRC is going to take that 6 years (assuming TPU time) and squish it down to maybe 6 months of actual full time credit. So basically you're starting from scratch when it comes to Active Federal Service, which is ok to, assuming you can do 20 years before you hit retirement age. Whatever you do, once you're in, take advantage of the schools (military and civilian) and field experiences you can. Don't be that guy that sits behind the keyboard and never goes anywhere. Keep yourself relevant. Good luck.

I would bet the most sought after are: 42A, 92Y and 79V. If you want to reclass in the reserves and then move to the AGR, those are probably your best bets.

The best odds of making SGM or CSM, is anyone's best guess. Maybe 79V?

If you want to come straight into the AGR, google "recruit the recruiter" and come straight from the streets as a 79R. That's arguably the most "needed" position. But you're not an NCO, so I'm not sure about that.

This is all anecdotal, if you will. I just got picked up as a 42A.

25U is OK and not uncommon in the AGR program.
The most common MOS in the AGR system is 92Y, followed by 42A.