We already had this question asked multiple time, just check adapter topics they have all.

Metabones is best in absolutes (if you have unlimited money or can pay for it from work). RJ is usually better than Mitakon, all else on the market can use same standard metal adapter but source of optics can be any (as it allows to save on cost), so it is impossible to tell, can vary from time to time even.

Thanks for the concise answer to this question Vitaliy! I've wondered the same thing.

My RJ lens turbo has worked well for me, so unless I would go to an "active" Metabones for current Canon lenses, I think the RJ does the job quite nicely for me.

Have to say that RJ was very helpful about it so if you have a problem no need to worry, also had a regular RJ adapter that was perfect and a genuine metabones speedbooster that was a bit loose so a bit the luck of the draw.

Well, they can write anything, it is marketing (some companies have up to 8 different abbreviations in lens name). And all optics are now designed in special software, most of the time location does not matter much.

I can only say that the second series of the Zhongyi is the best I've seen until now next to the Metabones.

Metabones is still the best, but you pay quite a bit for a relatively small margin of perfection. But watch out if all of your lenses will fit, my Contax 18mm for example doesn't.

https://translate.google.com/translate?depth=1&hl=en&rurl=translate.google.com&sl=ja&tl=en&u=http://dc.watch.impress.co.jp/docs/review/special/20140416_644431.html RJ seems to have the BAVEyes beat. Metabones still king. But resolving wise the RJ seems to cope pretty well but seems to worsen chromatic aberration/purple fringing behaviour the most. Too bad they didn't include the Zhongyi in the test. But people seem to really appreciate the latest version from Zhongyi, so I'm going to give that a go and skip the BAVEyes. I'll get myself an EOS to m4/3 for my Contax Zeiss f/2.8 primes and then add the K&F Concept C/Y to EOS adapter.