What's the best 4/3 body

What is currently the best 4/3 body with in body IS that a Canon FD lens can be adapted to? Or if there are more then one whats the list?

Since none of them will use AF or auto-anything, almost any body will do. I'd suggest selecting a body for your purposes and then mounting the adapted lenses to whatever you buy. Example: if you want the best video, a Panny GH series would be a good choice. For stills and action, an Olympus EM series would be good.

Any micro four thirds body will permit an adapter to take your FD lenses. If you are willing to change the adapter tube to each lens you might consider the ones by Metabones or Vello which slide on easily in my experience. FD lenses will be heavy so I would think of one of the larger bodies or one that has a battery grip. I have used FD lenses successfully until I decided that I like the in lens stabilization as well as in body stabilization. One that is currently a good value is the GX 8 which is solid and has body stabilization and a beautiful viewfinder image. Many good used models to choose from also. Depends on what style you fancy.

So I currently have a couple dozen FD lenses including a majority of the L lenses so The ability to function with them would be paramount. The suggestion of a GH-8 got me to take a look and since I am not interested in 4k video just still photography with my currently lenses. I took a look at the GH7 which with a Kit lens 14-42 looks like one heck of a deal.

An Olympus EM-5 MII would be high on my list. Large enough (with an accessory grip), to handle FD lenses, very good EVF, and world class IBIS system.

What level of functionality are you looking for with your FD lenses?

Well I'm pretty sure the level of functionality is a set level. Manual focus and either AV or manual exposure with Image stabilization being important on anything longer then 200mm Which I go to 600mm with my lens collection. I'm into mostly nature photography from forest floor and wildflowers to birds and wildlife. So 90% of my photography is not in a big hurry. And since I would be getting what ever Kit lens came with the camera I would be pretty well covered for the snap shots and stuff where AF might be a real advantage.

Focusing and exposure will be manual with FD lenses. Honestly, even though I have a GX8 I have no idea how the IS would function with adapted lenses. As I understand it the Olympus EM1 II has the best in-body IS, so that might be worth a look, IF it works with adapted lenses.

I believe the GX8 will allow 3 point IS on any adopted lens like the OP is looking for. The Olympus models like the EM5 and EM1 will allow 5 pt. stabilization.

If you don't need the robust weather sealing, the EM10ii has most of the features of the EM5ii and similar IQ. I use it often with FD 50/1.4 and FD 50/1.2 and sometimes an FD 135/2.8 without any issues. I also have a Nikon to MFT focal reducer that allows me to shoot Nikon AI lenses a stop faster.