I'm not sure how helpful this is, and I'm sure this will vary with your shooting style, but the lenses I have and found pretty useful are the following:
Note: The forum scales the images down so to look at them a little bigger open the image url directly.
1) SMC Pentax-D FA 645 55mm F2.8 AL [IF] SDM AW, very sharp and has a nice contrast. I hear some samples might be better than others, as is with all lenses. I'm quite happy with mine. Below are a few samples shot with it.
2) SMC Pentax-FA 645 120mm F4 Macro, Also very sharp (sometimes to sharp for portraits) and useful for many applications. I sometimes find the bokeh to be funky looking at some distances but generally this does not happen often. Some samples below, the one with the butterfly on the statue shows the bokeh issue I'm talking about.
3) SMC Pentax-FA 645 150mm F2.8 [IF], Nice lens for portraits, it's low contrast yet still sharp wide open, has a bit of a build in pro mist filter look at 2.8 but becomes very sharp at f 5.6 and higher.
4) SMC Pentax-A* 645 300mm F4 ED [IF], Out of all the lenses I have, this is the one that I use the least, but for some situations it's absolutely indespensible. It's sharp if you keep your shutter speed high while on a tripod, and mirror lockup is a must if you are under 1/500th of a second assuming you want critical sharpness. It's actually not to bad for portraits either if you are looking for a nice out of focus background, but it's nothing the 150mm can't handle at f2.8 as fa as bokeh goes. I tortured my father to be my test model one more time on this one:
This one shows a comparison between the 55mm and the 300mm from the same vantage point, this is not the sharpest shot but that's more my fault than the lens. You can definatelly resolve human hair from this distance if your technique is spot on.
5) SMC Pentax-DA 645 25mm F4 AL [IF] SDM AW, This one is a tough one to recommend because of it's price and limited use. I have a unfinished writeup about it on my blog I intend to finish soon. To summarize it, the lens shines in some very specific scenarios, it's razor sharp, has great micro contrast, is super picky when focusing and has some chromatic aberrations in certain situations that can be usually corrected in lightroom. It's definitely not a typical walk around lens because it's pretty big and heavy, and you have to be in some pretty tight spots to really make it worthwhile. I hear some good things about the 35mm lens that pentax has, and tried it at a store in Tokyo. It's probably a more useful and much cheaper alternative if you want to go wider on a tighter budget.
Wide open at F4
I hope some of this is useful, I have not tried any of the zooms but hear mixed reviews here and there. I'm sure some of the other forum members that have used pentax glass on film bodies can chime in.