"Who currently has the best 6538 case?"
That is a very good question.
Short story:
Having owned a genuine 6538 years ago (sold it too soon it turns out), I was not overly impressed with the watch in general when I owned it.
Q...Why?
A1...It was small and did not look much like a 'submariner'.
A2...The 1030 is not one of my favorite movements, I like the 15xx much better.
Looking back, I miss the price increase I could have profited from but do not miss the watch at all.
With that out of the way...
I have seen quite a few 6538 projects and very few appear to be genuine, even at first glance. Some were too big and some just had the wrong 'look'. It's hard to explain but easy to see.
Otoh, righteous 5512/5513 project watches seem to be more accurate overall...imho.
Still the question remains..."Who has the best 6538 case?"
My answer is there is really no correct answer, but my guess is the 'best' 6538 watch will be the result of a very good $$ case with good letters/numbers, spring wire bra$$ bezel, a super fine $$ 'gilt' dial, professional $$ 'ageing', and expert $$ detailing and assembly.
If you only want a decent 'knock around' 6538 with an ETA or A21 and a decent dial, the same advice applies, but without the $$ symbols.
I know this answer is no help to you, but it is a friendly caution about what lies ahead having chased a few of these dreams/nightmares myself.
Hopefully someone who knows more about aftmkt 6538 cases will have a much better answer.
Good luck!