Going along with your "take a gamble" idea: Helios 44, I've had a lot over the past few years and they are all good but the 44-2 with lots of aperture blades is very nice, almost as good as Biotar 58/2.
Less of a gamble but still "secret sauce": Tamron 300/2.8 gamble on eBay, ended up having great glass and great results. C/Y mount Planar 50/1.4: better than the Canon EF 50/1.4 and a lot of other manual focus 50-55mms. Rokkor 58/1.2 but that's not much of a surprise, really.
Medium format: Super Takumar 105/2.4 for Pentax 67, first edition gives great results. Maybe the later ones would have less flare but I can't really see how else they would beat the early 105mm. Had a Yashica D with triotar, surprisingly good. Extremely trashed and rough operating 532/16 with lots of fungus in the Tessar 80/2.8 lens. Threw a test roll in just to see what happened and the results are quite good, especially taking into account the age and condition of the lens, never mind the rest of the camera.