Opteka 85mm f1.4 - which adapter?

I would choose Canon or Nikon mount. Larger user base, and most likely greater resale value if you decide to sell it later.

 

Opteka, Samyang, Bower, etc lenses are completely manual with no electronic connections, so there should be no functional difference between the versions made for each lens mount.

 

If you can get one that fits some of the older manufactureres that aren't really around anymore (Minolta MC/MD, Yashica/Contax, etc.), you'll find a wealth of other reasonably-priced legacy lenses that'll also fit the same adapter. If your choices are just the ones that you listed, I'd probably go for Pentax for that purpose, just because there's less demand (and, consequently, lower prices in many cases) for their lenses.

 

Not true! They do make a Nikon version with the metering chip{they have two different lenses for the Nikon mount , with and without the chip!}. It wouldn't be of much use if you adapt it but great if your a Nikon user. The Olympus version is four thirds , full 4/3 not micro 4/3. The 85mm on a m43 body would be a 170mm equivalent to a full frame which makes it a medium tele. Get a good quality adaptor because this lens is not a light one.

 

Well, that is positively the last time that I recommend anybody get a Nikon anything!