"it depends". Dell is fairly good overall, on-site techs are outsourced subcontractors a lot so that can be a mixed bag, pushy sales.
Supermicro is good on a budget, not quite mature full fault management or complete SNMP or redfish, they can EOL a new line of gear suddenly.
Have not - looks nice though. Around here, you'll mostly only encounter the Dell/Supermicro/HP/Lenovo. I actually find Dell to have acheived the lowest "friction" for deployments.
You can get device manifests before the gear even ships, including MAC addresses, serials, out of band NIC MAC, etc. We pre-stage our configurations based on this, have everything ready to go (rack location/RU, switch ports, PDUs, DHCP/DNS).
We literally just plug it all up and power on, and our tools take care of the rest without any intervention. Just verify the serial number of the server and stick it in the right rack unit, done.
