I've used the following with very good results:
Fododiox EOS adapter + EOS to Leica R adapter:
--Leica R Curtagon 35mm shift lens, no vignetting, will support 2mm of shift before vignetting becomes a problem. Effectively a 28mm on GFX and sharp across the frame.
--Leica R 35-70mm f 3.5, vignetting below 43mm. Sharp across the frame.
--Leica R 100 f2.8 macro -- excellent general performance with no vignetting and strong macro lens performance
--Leica R 50mm Summilux f1.4: some vignetting but almost all cleared up using Lightroom lens profile + manual vignette correction
--Leica R 50mm Summicron f 2.0: more severe vignetting
--Leica R 60mm macro--great, light lens but vignettes
I tried a number of Canon lenses but anything 50mm and below (except the T/S 24mm II) vignetted (Sigma 24 and 35 f1.4, Canon 50mm f1.2, various versions of 24-70) and the hassle of not being able to change aperture made these lenses less useful than the Leica lenses, all of which have manual aperture rings, so I didn't investigate using Canon lenses more thoroughly.
Hasselblad HC to GFX adapter (Fuji's own, just released adapter):
80mm f2.8 Hasselblad HC lens: very easy to focus, no vignetting, very sharp across the frame and supports shutter sync at 1/800--tried it with Fuji 500 flash and was able to overpower the sun at 1/800 shutter speed using lowest ISO and f22 in bright sunlight. This is also the lightest of the HC lenses (63mm full frame equivalent). Apart from slightly different focal length, many shots were mostly indistinguishable from the new 63mm f2.8 Fuji lens but the newer Fuji has slightly more pleasing bokeh.
I also tested the HC 35, HC 50, HC 55-110, HC 30-90 and HC 100 f2.2. All worked fine. In terms of price/performance, for an initial lens to use when needing faster than 1/125 shutter / flash sync, I acquired the HC 80 as a good, inexpensive, general purpose lens. All of the others worked well (but range from fairly to quite expensive, even used). The HC 100 was truly stunning, which suggests that the forthcoming Fuji native 110 f2.0 will be amazing--and will cost far less than a used copy of the HC 100 f2.2. The "wow" factor of the HC 100 is must be seen.
A few other comments on using the Hassy adapter:
1) all the Hassy lenses did pass complete exposure information to the GFX--aperture (and, of course, shutter speed, and ISO).
2) For best use, one should change the lens adapter shutter setting in camera to "lens" rather than "body". Shutter performance is noticeably faster and quieter with the HC leaf shutter than using the native, in-body GFX shutter mechanism on HC lenses.
3) One aggravation using the HC adapter: one has to focus wide open and at relatively high ISO (indoors) to see focus accurately and then stop down (and lower the ISO) to take the shot. There's no automatic "focus wide open but shoot stopped down" mechanism, making the lens cumbersome and slow to shoot indoors. My awesome dealer investigated this and spoke to Fuji directly, who confirmed the lack of automatic stop down shooting, but said that the firmware was finished just prior to shipping and that he expects there to be a number of (unspecified) functional upgrades to the adapter with subsequent firmware revisions from Fuji--so we can, at least, hope.d
4) With the Hassy adapter and any other manual lens adapter, one has to turn off both the preview WB/exposure and the companion preview picture settings. The GFX EVF will gain up (indoors it can become very, very grainy) and this helps focusing to some extent. You'll need to read EV values to understand your true exposure and adjust aperture/ISO/SS accordingly. In many cases, even turning off both picture preview settings isn't enough--I find turning up ISO, focusing and then turning down ISO, while cumbersome, works fairly well and my hit rate is quite good. As I already noted, there is no "focus wide open," "shoot stopped down" function yet for the Hassy adapter. Using flash indoors requires exactly the same discipline. So far, when lowering ISO after obtaining focus, both the Fuji 500 and the Nissan 60A flashes have worked flawlessly to correct the exposure. And both work quite well up to 1/800 with the Hassy adapter.
I'd be curious to learn others' experience using the new Hassy adapter.