What are the good manufacturers of video conferencing system ranking

Personally, I feel that these are not bad: FMS, good meeting, full time. But have to let people evaluate which is good, have to test yourself to compare, after all, now the video conferencing system is very mature, the basic functions are similar, just the protocols used by various manufacturers and core technology is not the same, the most critical thing is that you have to consider that you are not only buying a system, but also a service, so the after-sales service is also very critical.