Mine is really, with no hesitation, collection highlighter.
It’s really THE reason of using MB for me: seeing all what I already have (recordings, mostly), including in albums I don’t have.
Seeing all things that I might want to acquire (see why with my second favourite feature) without adding redundancy.
It is not a genuine MB feature (it’s a user script), but it’s thanks to MB. And I really hope one day it will be part of MB, to save my maintenance time. And also to enable this, even on browsers that don’t support user scripts, like most mobile browsers.
My second favourite feature is ratings!