Depends what you mean by sincere. And I think it varies by individual, and is all relative.
My take:
Splane - sincerely convinced, also sincerely full of crap, and very arrogant with it
Loesch - I have my doubts about him. He may have believed it fully to stsrt with, but now is in too deep to allow himself to deviate from the path, even if he realises it's rubbish
Morris - perhaps the most likely candidate for insincere hanger on, in my opinion
Jackson - actually seems quite sincere to me, perhaps the most sincere of the current bunch. He didn't do well at the ARC, but at least he tried
Sanderson - who knows? Quite young and ambitious. Who knows?
Lett - this guy is just plain nuts. Who knows what he believes? Does he know? I think yes, I'd put him down as a sincere believer
Who else is there again? I forget
Oh yeah Herd
Herd - I reckon he's a true believer too
That's my take.
In historical perspective I reckon the most sincere believer ever on the Governing Body was probably Jack Barr. And the most likely candidates for people who realised it was all nonsense - Nathan Knorr and Lyman Swingle.