Favorite legacy lens to use on Micro 43?

Here I must completely disagree!

All brands have their stellar lenses and their dogs.

Some brands, like Zeiss or Leica, have fewer dogs than others, but they also have lenses where the price/performance ratio is even worse than usual... 

Each lens needs to be looked at individually. There are huge differences between similar lenses, even from well-known brands (don't get me started...)