I gave up on all the online services long before Plex settled on Musicbrainz. I think which one is better is going to depend entirely on each individual’s library. MB may have better data in some cases, and Last.FM in others.
My library is thoroughly tagged and I have local artwork for artists and album covers, and Plex is configured to prefer local data over online services. I’ll say that, since I got my tags whipped into shape, Plex, using the newer “Plex Music” scanner, has provided the most accurate results yet, for me.
The new scanner is more accurate in setting up my libraries, but it’s also pickier about folder structure and file names, than the old scanner. That kinda doesn’t make sense to me – better tag support should allow Plex to be less reliant on file/folder structure, not more – but that’s how it is (on the bright side, a picky scanner identified a lot of mistakes in my tags, so it actually helped me out).
Plex still has a lot of work to do, especially for classical music, but I’m hopeful.
