I've been through 2 Part 121 programs and 1 Part 135 program.

ASA's takes the crown. They spend lots of time teaching procedures, systems, regulations, etc. ASA's training program was geared for pilots with 250 hours and a wet commercial ticket (back before 1500 hours was required, so it might have changed since then). It was a long training program but a thorough one. We had a whole week of systems training and FMS training. We'd go through 1-2 systems a day, never more than that, and then hit FMS. It didn't feel rushed and there was very little "go home, teach yourself this and come back." Since it was my first 121 jet job, I think that helped a lot.

Compass has a solid training department, but you can tell it was designed for people with prior 121 experience. You get a disk, go home for a month, and come back "knowing everything." I did fine but my prior 121 jet experience was helpful. I would have had to work a lot harder if I was straight from a CFI or 135 job.

I also flew at Ameriflight, and their training program is outstanding. Not just by 135 standards, but by training standards in general, they take safety very seriously and make sure you 100% know your sh*t before you go out on the line. Their training department is filled with good, knowledgeable people.