MNX is probably good but they are taking an awful long time to get the job done. They ran into the same roadblocks as any format, there's too many competing needs and too many opinions. MIDI 2.0 has also had some really interesting talk about embedding notation data inside the wrapper to make it an "all-encompassing" music format.

I'm not as familiar with MEI but I don't believe it's any better for preserving music engraving data compared to the musical data itself.