manual film slr with brightest viewfinder:

[Joosep]

The Bessaflex. Hands down.
Used Nikon, Minolta, Fujica, Minolta, Pentax, Canon,... manual slr-s.

But the difference is really small between the tops.

 

[Kolame]

My Minolta XD-7 is fine, but the viewfinder of my fathers X-700 is just awesome!

 

[santino]

Leicalfex SL - king of brightness 😀
Contax RTS II

 

[bastian a.]

Leicaflex SL/SL2
Leica R8/R9