I don't really think that it existst any "Crowded" non lag highrate servers right now.
Sorry but i can't answer your question since 95% of the private server laggs and are full of hackers right now
Im playing on my guilds PS and on Maplelegends even if maplelegends laggs it's easy to get levels.
Try to search for : Maplestory private server on google xD
And try to AVOID v.55 servers if you want to play legit :P