Top tier is still MarkMonitor. Last I spoke with them, they had a five-figure minimum spend, but the per-domain costs are competitive. That cost buys you proper named support contacts, etc.
If you look up the whois for microsoft.com or yahoo.com, that's who you'll find.
Five-figure minimum spend sounds pretty expensive for the vast majority of businesses out there. Of course, just a drop in the bucket for major brands.
Definitely. I don't use them for my personal domains, of course.
But as others have pointed out, there's basically zero margin on simple domain sales. So if you want proper support, you need to go to someone who bundles it with other enterprise business (e.g. AWS), or who makes it their whole business (e.g. MM).
If you look up the whois for microsoft.com or yahoo.com, that's who you'll find.