I've posted about it before, but my EF 200/2.8 mk. II has fantastic image quality. The only thing missing is IS, though with any luck, IBIS in future RF bodies will help to make up for it.Canon EF 200mm f/2.8L IS USM.
I really like my old EF 200/2.8. It is fast, light and affordable (700€). A new version with IS in the price-range of 1000€ would be very nice. And it would be a great addition to my 85/1.4L IS