I think there are enough info for Nikkor so I won't repeat.
For M42 I'd recommend Carl Zeiss Jena Sonnar 135mm f3.5 MC. You may just get it for $100 because I sold one of my mint cond for that.
Sharp enough and has very pleasant buttery bokeh.
Jupiter 9 as mentioned above is sharp stopped down but it also has that 'helios' like distracting bokeh you don't like.
I wouldn't equate the Jupiter 9 bokeh with that of the Helios. J-9 is a Sonnar copy and to my eye has the smooth Sonnar bokeh, as does the J-3. However bokeh is in the eye of the beholder, so best to see for one's self.
Thanks, the info here is amazing. now I can widen my search. Actually it's not that I don't like the bokeh pf the Helios 44 ( i actually use it quite a a lot), But for the look I am going for I want it to be "tame".
Do not exclude the 100mm and 75-150mm Nikon Series E lenses from your search. They may do not have the prongs (also called “rabbit ears’) that allow the Nikkormat’s built-in light meter to work; however, if you are using a hand-held light meter, you do not need the built-in light meter. Also, if you are shooting portraits using flash under studio conditions, you do not need the built-in light meter.
Actually I use a Series E 50mm in my nikkormat, I use the DOF button for stop down metering. Plus, Since I started with film a few months almost a year ago all my cameras do not have a meter (busted). The Nikkormat I have now tho has a working one, a nice bonus. I'll look into the zoom, I've always stayed away from zooms (made me very lazy), but I'll consider it.
Another thought. How about the Tamron Adaptall-2 135mm f/2.5? I have one and although I haven't had the opportunity to use it in anger yet, the test shots I did were very encouraging. Nice colours and bokeh, and the added bonus that it is cross-platform with the correct adapters.
Mamiya 645 Super | Nikon F4/F100 | Minolta Maxxum 9/Dynax 7/X-700/X-500/XD7/SRT-101 | Pentax Spotmatic | Canonet QL 19 (GII) | and a whole bunch of glass