Brightest / easiest to focus SLR
It has been noted that the camera with the brightest finder is not always the easiest one to focus. I find the Minolta X-700 with a grid screen to be both bright and easy to focus. It is easier for me to use older M42 lenses with an adapter than to use them on any of my M42 mount cameras. With the grid screen I don't have to worry about getting the subject in focus with a distracting focusing aid and then recomposing. I can focus on any part of the screen. It is not likely that any third party company made focusing screens for the X-700 which were better than the ones Minolta made. For very close macro work or with very slow lenses I prefer a plain matte screen. I find changing screens in the X-700 annoying so I leave a grid screen in one, a plain matte screen in another and the standard screen in the rest. Where Canon manual focus cameras are concerned, if I use an L screen in an F-1 or F-1n it is very close in brightness to using an F-1N with the same type screen. The older Konica Autoreflex cameras did not have the brightest finders but the T3 is not "1960s technology." The T3 came out in 1973. I have a T2 with transplanted Nikon E screen (grid) and also an FT-1 with a transplanted E screen. These are handy for macro work and with slower lenses so I don't have focusing aids in the center blacking out.
Before I had Nikon FE and FE2 cameras I got an N2020 for macro work. At first I had planned to replace the standard screen with a B or E screen but I found that the standard screen was fine. The N2020 has fairly primitive auto focusing but I don't use its AF so that doesn't bother me. The FE and FE2 have viewfinders which are not as bright as those in the Olympus OM-1 to OM-4series but they seem more contrasty. It's funny about the Olympus cameras. All five of my OM bodies have non-interchangeable screens. I have three OM-10s, an OM PC and an OM 2000. The OM-10 has a pretty bright viewfinder. I may have one of them overhauled to get a grid screen put in. The OM 2000 is made by Cosina and has the same fairly good viewfinder as the other Cosina-made SLRs I have.
Other SLRs with nice finders that I have include the Minolta SRT 201, Nikon F2, Canon FT QL, Canon FTb/FTbN, Nikkormat FT3. Which ones have less nice finders? Pentax Spotmatic, Pentax Spotmatic II, Mamiya 500 DTL, Mamiya ZE, Mamiya NC1000/NC1000S, Miranda Autosensorex EE, Vivitar 220SL.