Mine would be a Chinon MC 135mm f/2.8 in M42 mount, got it on ebay for $60 shipped.
Stopped down below f/4 it is amazingly sharp, but I can't focus the thing worth a damn. I don't have a problem with other MF lenses, it's just something about this lens that causes me to miss-focus 80% of the time.
Also, I need an inordinately fast shutter speed to keep it steady, SR doesn't seem to have much effect on it...
I keep it because it is a beautifully constructed metal lens, the pop-out lens hood is adorable, and the bokeh at 2.8 is very creamy and smooth - a unique type of bokeh among the lenses I own.
Here is a sample.
If I ever need to show off a well-built lens to someone, this is it.
A K-mount lens I never use anymore is Takumar-A 70-200 f/4, I bought it a few years ago as an alternative to my Sigma 70-300 (which I sold about a year ago), it is marginally sharper than the Sigma, but still has the same problems with PF. I keep it because I've taken some memorable pictures with it, and it is my only "one-touch" zoom and focus lens.