If you don't mind annuals, I like peas & beans -- stacking functions -- fixing nitrogen, generating biomass (chop & drop), edible (pick what you want), and reseeding (about the easiest
seed you can save). I like to scatter some oats so they have a little support. Oats don't stack quite so many functions though.
I'll second comfrey and burdock, just chop before they make seed. Horseradish always comes up in these contexts and it's on my list of things to try.
A short-rotation annual is buckwheat. Attracts beneficials, suppresses weeds, makes decent biomass. Reseeds if you let it go that far, but doesn't make itself a nuisance if you chop it before seeds form.
Lastly, look around you: what do you see growing vigorously at forest edges? Are there plants that you could dig up (or gather seeds from) and add to your garden that are suitable for chopping?