Originally posted by yusuf May be easier question would be then, what are the bad 135mm? Some Pentax 135mm are not reviewed so positively.
Here is a search of Pentax 135 reviews. The only Pentax prime that gets less than an 8.00 score is the Takumar Bayonet 135/2.5 with two entries: 7.83 and 7.50. That second entry is curious: 3 reviews, one with no rating, with with an 8 rating, and a 7 rating by someone who had never owned nor used the lens! Toss that out and include the others with the 37 reviews in the first entry, and the score approaches 7.90, not exactly a 'bad' rating. And the Tak Bayonet's main shortcoming is that it has MC rather than SMC coatings, so users shouldn't point it into lights without expecting flare. So it's still very good optically.
Here is a search of 3rd-party 135 lens reviews. Of those that score 7.00 or less, most have only one review, so it's hard to say whether it's bad design+production or a bad copy. Luzers include: Soligor 135/2.8 (but were many makers of Soligor lenses); and Ricoh 135mm f2.8 XR Rikenon. Yet elsewhere I've seen rave reviews of each. And these small-sample statistical results aren't really significant. Oh bother.
EDIT: As Lowell mentions below, M42 Ricoh's were also the products of various lensmakers, so we can't ding them all for the shortcomings of one variant.
I can look at my own database of lenses, at the Pentaxable 135s I've sold or hope to sell and why, and I come up with:
* M42 Super Takumar 135/3.5: beat-up
* PKM Chinar 135/2.8: unexciting
* M42 Hanimar Automatic 135/2.8: unexciting
* M42 Hanimar Tele-Auto 135/2.8: beat-up
* M42 Porst Tele MC Auto D 135/2.8: fogged
* M42 Rexatar Automatic 135/2.8: fringing
* PKM Rokinon Automatic 135/2.8: unexciting
* EXA Schacht Albinar 135/4.5: sticky focus
* M42 Sears Auto 135/2.8: duplicate
* PKA Sears Auto MC 135/2.8 'macro': fringing ** this is the 'aura' lens I should have kept
* M42 Steinheil Culminar VL 135/4.5: botched rebuild
* M42 Sunset Auto 135/2.8: fringing
* M42 Suntar 135/2.8: unexciting
The Rexatar and Sunset were the only ones whose optics I didn't like, or at least they failed my PF test. But PF can be fixed in PP, and they buyers never complained. In fact the buyer of the both the Sears 'aura' macro and the Sunset both commented that they liked the sharpness. So only the Rexatar was a real luzer. All the others were either beat-up|damaged, or a duplicate, or just not exciting. Even *I* get bored by some lenses.