Engines are all about a matter of preference.
What level of graphics do you want? Low/Mid/High/2D/3D/text-based.
What is your favourite coding language? C++/Python/etc.
Input method(s)?
Use of chat, and to what extent?
Do you wish to program the networking side yourself?
I quite liked what I saw in Panda3D, personally. It's been used by Disney to make two commercial MMOs, it's open source, easy to setup using Python (even I got test programs made, by myself), the networking side is there (there's even available Python code for the chat interface), and the community and programmers were extremely helpful in #panda3d on freenode.
The PlaneShift source code already has a lot of the same stuff built into it, but of course as Underthemoon said, it was built for PS, and has advantages and disadvantages in that.