Hi Dennis,
It's mostly a matter of taste and convenience, so everyone's mileage will vary. We could be more helpful if you mentioned what subjects/styles/intentions you're selecting for.
Taste:
The new Fuji lenses are fine examples of modern resolution and sharpness, way out to the corners. They were designed for the GFX's sort of digital sensor (and presumably its successors).
If you're the sort of person who regards such lenses as rather 'clinical' and you're using MF with an emphasis less on resolution and more on dynamic range and smoother tonal transitions, then older Zeiss, Mamiya, Pentax, etc., are just as welcome, particularly for their lower contrast. You have to use these with manual focus and choose the older ones with manual aperture ring (unless you have MF Contax lenses to use with Fringer electronic adapter).
Convenience:
Do you need something longer than 110/120mm?
Do you have a legacy of MF lenses you can use with an adapter? If you don't mind manual focus and the aperture can also be set manually, then try these before you invest in more than one Fuji lens?
Do you have adaptable full frame DSLR lenses over 50mm focal length – long enough to use without vignetting?
Do you want to use FF TS/PC lenses with wide image circles? These communicate the aperture setting electronically, so you need an electronic adapter (now available for Canon).
Remember that when you use a lens designed for a DSLR and then use an adapter, you're in effect adding the length of a mirror box to your mirrorless camera. This will usually be bulkier than a lens designed for mirrorless in the first place.
Kirk
PS, my personal preference so far:
Taste: I make mid-sized BW landscape prints (up to 17x22" paper). I skipped the Fuji zoom lens because with zooms I tend to become lazy and shoot from the most convenient angle rather than the most carefully considered one. I'd been using Leica and Zeiss lenses with a Kolari-mod Sony, so I moved to MF/GFX not really for more resolution, but for possibly smoother tonal gradations with higher pixel pitch.
Convenience: I started with Fuji 63mm, which was all that was available besides the zoom. I began by stitching for wider angles, and then decided to try some older primes. I would have preferred Zeiss Contax 645 lenses but they were harder to find and more expensive. Second choices were 645 Mamiya N or Pentax-A (smaller than Pentax 6x7). I initially found Pentax 645 35mm and 55mm in 9+ condition, and their IQ feels just right to me, so I didn't look further for Zeiss. For wider angles I still stitch. I tend to use the Pentax lenses and leave the Fuji 63 home. I have the Pentax 645 45mm too, but it's a little less successful from the standpoints of sharpness and flare, so I might try the forthcoming Fuji 45.
I'm happy with what I have, but would have preferred to have stumbled upon a set of Contax 645 Zeiss lenses. I have Zeiss Contax C/Y lenses for Sony, and I do like that ol' fashioned Zeiss '3D' rendering.
I've heard that tilt-shift adapters for other MF lenses might be available in the not-too-distant future. I'd like to one this when the time comes, so I could shoot more of my landscapes at f8 instead of smaller apertures.
K