What advice do you have for someone reclassifying their MOS in the National Guard?

Well, let me put it like this:  Do you want a meteoric career on active duty?  If the answer is yes, try to get into civil affairs.  Their pickup rate from SSG through MSG is through the roof.  11Z pickup rates hovered around 9% this year, Civil Affairs easily quadrupled the Army average of 10-11%.
It is a demanding MOS with a high threshold for initial entrance and can feature a lot of nontraditional deployments, so this is definitely something to talk over with the family.

On that note:  I don't have a favorite MOS.  While certain aspects of specific MOS appeal to me, I think I only have MOS I would not want to be in right now.  CBRN comes to mind.

I've been an AD 19D, a NG MP, and now I'm a NG 15T. I enjoyed being a Scout...when we were doing Scout things... which, in my short time, was about 75% of it prior to 9-11.

I enjoyed my deployment as a MP. We had a PTT mission which meant we didn't deal much with detainees and didn't have to worry about prisons or any of that nonsense. It was all combat patrols.

15T is my favorite though. It helps that it's been my dream to get it. It's a good job and I've encountered a lot of good people. It also transitions well to the civilian sector. Promotions are slow though because of the very limited number of slots.

In the end it really depends on what you're looking for out of your MOS and your career. Do you want a good transition to the civilian sector? Do you want deployments? Does the speed in which you rise through the ranks matter? Is AGR or the Technician program something that you're interested in? All things that you need to ask yourself.

If you are interested in deploying, MP. The system was running out of MPs and security forces from all branches because there was so much road to patrol and so many detainees to detain and detention facilities to run, there just were not enough units to go around. Artillery, ADA, even Infantry units were sent to Ft Dix to train as MPs - mostly on the detention side - prior to going to theater. We even had an Airborne Infantry Bn from the 82nd running a detention facility (weren't too happy about it, but we needed the manpower and at that point - 2005 - there wasn't much ground combat going on).