Pentax Spotmatic.
If you're serious about the m42 mount, eventually, you come around to the Pentax Spotmatic line....and when you do, you'll wonder why you messed around with anything else. The Spotmatic F is probably the best choice for you but, the SP-II or SP-1000 are also excellent choices.
Do yourself a favor and try a Spotmatic.
What Brad said.
Cholelntpot,
I started collecting M42 lenses
after i discovered the Spotmatic.
The Spotmatic is close to an ideal manual focus camera. Imagine a Pentax K1000,
with higher build quality,
with self timer and DOF preview,
with same great ergonomics and VF,
with a prettier top,
and M42 mount. There it is.
The Spotmatic II improves with wider metering range and a smoother shutter action.
The Spotmatic F is the ultimate, it allows open aperture metering with S-M-C and SMC takumars.
AE operation is possible with the Pentax ES and the Chinon Memotron series. As well as the strange Yashica Electro AX.
There are some good fujicas as well, but I never felt attracted. I had a ST605n with the following difference to the yardstick (the spotmatic)
+ quick SPD metering
+ smaller
+ lighter
- only 1/750 top speed
- ugly
- lesser build quality
- the worst of all: shutter vibrations. A rough shutter.
There are two pluses to the Spotmatic that are seldom mentioned: The ergonomics and the VF.
The ergonomics are great. Your hand fits the camera as a glove. Everything falls naturally into place.
The VF is great. It isn't particularly bright or big, but it allows you to precisely see the focus point, has low distortion, good contrast and good eyepoint.
I used to own many Canon and Nikon classics like the F2, F3 and F-1 models. The Spotmatic series made me a Pentax fan as well. And the build quality of some M42 takumars is probably the best to come out of japanese optics makers.