>>6822from my experience (over 3000 hours playing wow on private servers), retrowow is actually pretty great. it has 5-man raids instead of 40-man, so it would be WAY easier to organize. I also have a fully decked out character there, so I could help anyone new who wanted to get gear.
There's also Everlook, which is currently the best option for a blizz-like classic experience, they recently made a new realm (completely killing the old one, where I had lots of characters, :( one a pretty well geared 60) where they're currently at the blackwing lair patch.
That's for classic servers. As for the others, it's pretty much one major server per expansion. So you're cucked in terms of options if you dislike the userbase of any specific server. I for one like Firestorm, as they have a server for every expansion, and are usually the first ones to release a (way less buggy than others') version of the newest expansion.