Originally Posted by pentaxman 
I respectfully disgree - The Rikenon XR 135/2.8 fits this category well. It is sharp and contrasty from F2.8 on and extremely well built. It even has a built in hood. These can be found for less than $50. I ran some lens tests against some other 135s (not the Vivitar) and found it to be by far the sharpest lens. It also displays almost no CA even wide open. I have seen APO lenses with more CA. A very high quality prime.
PentaxMan, I respectfully respect your right to 'First Right of Respectful Denial'
The Rikenon XR 135/2.8 is a fine lens and it has many of the qualities of the Vivitar and other leading leading 135's. But it is 'Rare as Hens Teeth' by comparison. Where as the Vivitar is so common that some people use them to tie down Helium balloons for the Forth of July Parade.
P.s. many as in most of the 135's have the built in Hood. The Vivitar and the Tair both have the built in hood...
It made me smile that I made a little dig at all the Pentax folk recommending Pentax lenses, and the first one to disagree with me is named PentaxMan and ironically recommending a Rikenon...
The rikenon is one of the better ones, and is a very good option as are many of the of the others mentioned in this thread. Other than the 50mm length, no other focal length has so many great old options...
With my tongue firmly planted in my cheek, and all of my respectful kidding aside...
Yours truly and correctly,
TairMan