rolleistef
Well-known
manual focus minolta with their accute matte screens.
My vote goes to the Japanese Contax SLRs followed by the Rolleiflex cameras (although not the original SL 35).
They are noticeably brighter than other cameras I've used.
I don't know about brightest, but best? F4 hands down. Everything in view but out of the image field, lighted if you need it, uncluttered by focus areas.
Usually, the manual focus slr's have screens optimized for contrast and "snappyness" whatever that means, while the AF versions are as bright as it gets.
This is also the reason why it is not too easy to focus manually an AF SLR.
I got a minolta x700, its really really bright, truly excellent.