I generally think any lens is an educational opportunity. A bad lens can teach you what to look for in a good lens. But that takes lots of time and some money, so here's my opinion:
Takumar-A 70-200mm f4 zoom. Crappy performance and it feels broken. I would stay clear of all Takumars with bayonet mounts because they were meant specifically to be cost-cutting lenses and not awesome. A few have decent reputations but more like "a good performer for the price" instead of "the best lens in its class". Today the price difference is pretty small between a Takumar-A 70-200mm f4 and an SMC Pentax-A 70-210mm f4, which is a good lens.
Mirror lenses, mostly 500mm f8. Some of these are not even cheap, though they are cheap for a 500mm lens. Even the "good" ones are difficult to use. I like my Tamron but it's mostly on the shelf.
SMC Pentax-M 50mm f2. This lens is not bad but again, it was made to be a cost-cutting lens and the price difference is not worth it. The coatings don't really look like SMC either. If you buy a lot of used stuff you might end up with one for free. There are lots of better or more interesting lenses at 50mm; you don't want to settle for this one.
Cheap third-party manual focus lenses at 28mm (f2.8), 135mm (f2.8) and 70-200mm (f4-5.6), especially with department store brands like JC Penney, Sears or Focal. Same reasons as above, meant to be cheap. A handful are good but it is more fun to get a really good lens at those focal lengths.
Thinking about my lenses more, maybe the Pentax-DA 16-45mm f4. This lens was one of the first lenses available for APS-C digital. With no competition, it looked pretty good. Now there are lots of alternatives, so it's discontinued. Mechanically it's not robust, so I'm not sure I'd trust a well-used one. Lots of people object to its weird reverse zoom construction, physically short at 45mm and long at 16mm. I suppose I'd get one for a very low price if my needs weren't challenging for this range, but it's such a common range so that's unlikely.
Last edited by Just1MoreDave; 08-19-2021 at 08:40 PM.