For sh*t and giggles I logged onto the top of the list for high pop PvP servers and had this ping to start with a noob and later went to a big 71ms. Of course there are a boat load of servers so some may be worse than others. To date I have tried playing six, at various times. The best MS has not always been on the lowest population server.
Largest problem I have had on any server was one night on an RP server I was getting a lot of disconnects. They knew about the issue and fixed it.
Heres a great link for US servers. Lists server ruleset pop etc etc:
EDIT: Thought I would add this. I tried "Bleeding Hollow" this eve for four hours. Saw MS of 20-104 tops. Guess I am just lucky. It was in the top 3 high and is supposedly the most populated server. According to Warcraft realm.