Which heroes are considered main tanks?

Which heroes are best on which maps?

Which team comps best fit each main tank?

Specific tips for the main tanks are appreciated too!

Also, a few Reinhardt questions. I often find myself at the mercy of my team, them saying "Rein use shield more" and "Rein attack more" all at the same time. How do I find a good balance between shielding and attacking?

How do I work well with off-tanks that don't synergize with Rein?

What maps should you never play Rein on?

What are counters to Rein I should look out for, and what counters those counters?

EDIT: Forgot a question, how do I free cam as Rein while shielding? I've seen it before and it looks helpful, but I've never been able to find it.