Favorite legacy lens to use on Micro 43?

There might be but they would be expensive, not much sense to buy, better to put money towards native lenses. Also image quality can be questionable for legacy lenses shorter than 40mm. Starting from 40mm and longer the lenses became more or less telecentric so they work better on digital sensors.

If I was into legacy lenses my favorite would probably be Canon FD SSC 50mm F1.4, then maybe Pentax Super Takumar 50mm F1.4 or some of its variations, and Minolta MD 50mm F1.4 is also good. I formed my opinion based on photos I saw on the internet. I don't have these Canon FD or Pentax lenses but from what I saw these were some sharper legacy lenses. I have Minolta MD 50mm F1.4 but I stopped using it once I got Olympus 45mm lens. And I had good luck with Russian Helios 44-2 58mm F2.0 which is also decently sharp at F2.0. But there may be sample variations with Russian lenses, I'm not sure.

I also had Minolta MC 58mm F1.4 lens - this was too soft, Sears 55mm F1.4 and Chinon 55mm F1.7 (supposedly built by Tomioka Japanese company, commonly considered a good stuff) - nothing special, just not sharp enough.

Another interesting legacy lens is Canon FD 135mm F2.5. It's decently sharp and it can be bought for less than $80, I think.