Thoughts on several of the lenses mentioned in this thread...
F 50/1.7 - a few years ago I had the FA 50/1.4 and was disappointed in it's performance wide open. Many older 50/1.4's suffer from poor sharpness wide open, so for many the 50/1.7, already sharp at f/1.7, is a far better choice.
Helios 44M 58/2 - it's known for its swirly bokeh, but it does resolve a good amount of detail and has it's own smooth and mellow rendering. Also, easily-provoked veiling flare and maybe even light leaks!
Character!!!
F 35-70 - so well-known for a cheap and hidden gem that some folks think it is as overrated as a Duke University basketball player. I like my copy a bunch, very sharp and nice color.
FA 24-90 - a year or two ago it was so highly regarded that prices on the used market approached $400!
Looks like things have calmed down a bit. From what I've read there is quite a bit of sample variation.
Now for my nominee: the oft-maligned Tamron 70-300 f/4-5.6 1:2 Macro. Purple fringing in the highlights! Occasional purple fringing in the
midtones Slow AF. Pentax has the 55-300. But! It has that 1:2 macro mode. And that is very, very nice if you like to photograph butterflies, dragonflies and other large-ish insects (and spiders). And the Tammy is
sharp when used as a close-up lens... except in this case "close up" is more like a meter away, and that's really great if you are shooting things with stings or flapping wings.
Oh yeah, add the Raynox DCR-150 close-up adapter and you get a
real macro lens.
And have I mentioned that the Tammy is stupidly cheap? It's frequently found (used) in some variation - Tamron itself has a couple different models of essentially the same thing, and there's a Quantaray-labeled copy as well - for $50 or less.