Pentax 10-17mm f/3.5-4.5 fisheye, APS-C AF zoom, 150-200€, amateur use.
Reason is, I already have my lineup down ok, what I miss are only specialty lenses, for those times when you want a particular effect, and it can't be replicated in post.
I already have a Helios 44K-4, which I try to use sparingly (
), a rectilinear UWA, a teleconverter with helicoid that I can use for macro up to (I found) more than 4:1 with a certain trick (hehehe), and as a 2X teleconverter proper, so I can go up to 400mm (600mm equivalent FoV on crop format).
All this quite on the cheap, but working as intended nonetheless.