I've never come across Support like 20i. No matter what time of day (or night), every issue I've had is usually responded to and often resolved in minutes. Incredibly knowledgeable and helpful, UK-based staff so no misunderstandings or language problems and genuinely nice people.
Oh, and the actual services they provide and their interface, customisations for resellers and just about everything seem very well thought out and work brilliantly.
I've not had experience with other reseller accounts, but these guys seem to be setting the bar very high. I can't believe there are many companies out there that give you this level of service.
I can't recommend them highly enough. A from me!