...not because they are famous, costly or have achieved cut status, but because you find them interesting?
The 203mm Optar is one of those lenses for me. Why? Because the 203mm Ektar gets all the 203mm love (and they are great lenses---one rides aboard the snout of my 5x7 Speeder.) Still, I'm entranced by the notion of the reclusive illusive Optar.
Or how about a 270mm G Claron? There are 240s and 305 G Clarons seemingly all over the place, but what became of the 270mms?
Another lens would be a Darlot Hemispherique Who the heck shoots with a Darlot Hemispherique? How do you even pronounce it after two beers? I suspect it just has to be a magic bullet lens. I can imagine this conversation spoken above the sound of the roaring (Que the Erroll Garner dissonant chords) surf in Big Sur 70 years age:
E.W.: "Ansel, quickly! Bring me my Hemispherique!
A.A.: "Edward, you left it at home on the kitchen table with those peppers."
Which lenses would you like to play with, just for fun?