Do you ever regret your choice of job in the military (MOS, AFSC, etc)?

Not at all. I was placed in my MOS because of the needs of the Marine Corps. I hated it at first, but I grew to love me MOS. I had initially signed up for the Legal career track, but I got put in Maintenance Records instead.

No two of my MOS's have come in handy over the years.

The Army involuntarily re-classed me twice at the 3 and 10 year career points. You learn to let go of regrets when you're learning a new job, attending new schools and being put in strange new leadership positions.

I don't regret my choices, helping people has always been part of my life whether it was medical or CA, I just wished I had learned about CA long ago

I don't know if regret is the accurate term, but there is a large part of me that wishes I had done something different. I went in as an Engineer. To be completely honest, I never anticipated the military as a career and was only thinking of job skills for after the military. I learned a lot of job skills in the area of construction equipment operation among a couple others. I was grateful for those. I had briefly entertained the notion of reenlistment during which I considered reclass into another MOS. There was just something in my current MOS that was lacking and still can't put my finger on what it is. Whatever it was, it, along with other reasons, sucked out all my ambition to stay in the MOS and even the military.
It's kind of a shame. I had already taken the DLAB and scored high enough that they asked if I would consider reclassing to linguist. Problem was, that really didn't sound all that exciting to me so I passed. I don't regret it that much, but I also failed to see that maybe there was another occupational specialty that would be more my speed.

No.

At times I sometimes wish I chose OCS instead of enlisted 11B when I joined. I'd be a Captain by now. That said, I wouldn't go so far as to say I regret my choice.

I do wish I had chosen something more interesting... My MOS stopped being challenging after a year or two... A monkey could do our jobs... It's a lot of hard work, but very little brain activity...

I joined the US Army at 21 and stayed for the next 5 years and 9 months; supply school and a mechanized infantry assignment. Then Fort Gordon and Fort Bragg. I decided that I could be an officer. 30 years later I retired as an Engineer Lieutenant Colonel. No regrets - they are a waste of time and energy