PWLMS (Perfect World LMS)
This and Bl00d Dragon are the only private servers that I've found to be near the top of almost every private server ranking list. This private server has been around for about 2 years (I think) and seems very professional.
You can patch the PWI client with the LMS patches and jump right in the game!
I have yet to figure out what the LMS stands for...
Credits for PWLMS (Perfect World LMS)
Me for submitting it, I guess.
And of course the developers of this private server!