Which Avionics MOS would be best for someone considering a civilian career after serving?

This is a difficult question because you are basically asking a group of strangers "Hey, what job do you think I will enjoy for the next 10-20 years that I could make a career out of while in/out of Uniform?"

The only person who knows what you like, dislike and want to do for a career is you. You can't expect somebody who doesn't know you to decipher that you would prefer airframes, avionic tech, engine or a crew chief position when compared to each other. As anything in aviation it depends on the respective airframe. There are certain things that overlap but if you have aviation experience combined with a bachleors degree there are alot of companies that would snatch you up. But at the end of the day its what you like to do.

Avionics can be tricky in the civilian world; a lot of places still want an electronics engineering degree with experience in specific areas that they are working with. some jobs in Aviation are less desirable while in but offer more post service such as Maintenance administration. Then there are jobs that cross multiple industries when you get out such as Machinists and the always desired Non-Destructive Inspection,

I don't know if you are dead set on Army Reserve, but Airforce Reserve technicians usually have aviation positions where you are a reservist but also have a full-time position doing the same job.

Go for 15T (UH-60 Helicopter Repairer). It's a good choice for a civilian avionics career, and you can get certifications like A&P in the military.