best is probably never found
The best person for you is the person who can communicate with you, convince you and help you.
I have had 1 paid for session and seen a few others going on, been in groups where a guide is coaching people and general other riders chipping in. My missus has done a few with different people and has had different experiences with all of them.
The coach needs to have skills but doesn't need to be the best. Some of the "Big Name/Racers" might not be the best coaches - you can't learn much from the guy who explains landing a jump as "You Just Do" etc. some people can be amazing riders but not be able to tell you how they do it or work out how you should do it.
I did a 1 on 1 session with Craig from Cyclewise (Whinlatter) sound guy, great coach and rider. I have ridden with a few of the guys who coach up there socially and it's great to hear some advice at the end of a descent or climb. For that I would recommend them as good guys, down to earth and great at communicating with a good course structure.
On the subject of what do you do on a course? If your going to get the most out of it you need to know what you want from it. If it's a group beginners course are you at the same level as the others - if not it could either mean you get lots of extra help at the expense of the group or not enough. Sometimes mates are not the best people to go with as you don't always need the peer pressure element.
If you can afford a 1 on 1 or 2:1 course with someone with the same needs/ambitions then that would be my preference. Spend some time thinking about where you are, what you can't/won't/don't do and what you would like to do. My session was in the run up to the mega so was focussed on flow, speed, racing & efficiency.
Final thing, if your going to invest in the skills invest the time to practice them. Turn up to rides early and get some extra practice in round the car park etc.