my vote, after about 7 years of casual use, goes to the Pentax 645n. These cameras are underappreciated and underpriced. You will be hard pressed to look through a better finder. Although the eyepoint is not stellar, everything else about this finder is. Want to actually evaluate depth of field and subject separation on the focusing screen? This is a functional SLR. Also, I think one would struggle to find a better arrangement of controls and functions. The lenses are top notch, and the dual portrait-landscape tripod mounting is super handy.
I give my SRT 101 the runner up. While I don't think these are particularly underappreciated, they are vastly undervalued. I paid a premium ¥10,000 for my pristine black paint body and feel like it was a bargain. Compare it to a popular camera like any of Nikons FM or FEs and you will find it head and shoulders above in terms of build and materials. A high watermark for Minolta.
Honorable mention goes to my Olympus Pen S sporting the Zuiko 3cm. In contrast to the SRT these cameras are not undervalued, as clean, good functioning copies still fetch a respectable price, but they are definitely underappreciated as functional shooters. 80 frames a roll with a no-nonsense, ultra compact form factor. The finder and brightlines are brilliant, and the short lens offers enough depth of field to not feel hindered by the lack of rangefinder focusing. Further, the half frames from this camera leave nothing to be desired in terms of image quality for the snap-shot style of shooting this camera excells at.