My primary goal is the sharpest I can find that will cover the frame.
Is it just cost that has you looking at C mount primes? Like I think others are saying, the Switars can actually be quite sharp. But the barrel is really small if you need to pull focus and C mount is no good if you are swapping out lenses quickly or checking the gate a lot.
Cooke Kinetals (Tyler Hobson) may be a way. Very sharp but less contrasty than Zeiss super speeds. Designed for standard 16, but I think only the 9mm vignetts, the 12.5mm may cover, maybe Dom could confilm or qualify that. Problem with these is that the lens mount itself rotates in the camera port, you have a light smear of vaseline etc in the port. The lens mounts are normally Arri S and there are quite a few that sell on eBay, unfortunately the sets are normally broken up to sell. I think maybe the prices are rising a bit. We may hear some complaints on the forums about how to get vaseline off one's BMPC sensor (smiling).
MK I Zeiss super speeds are the next option up. Always seem to be Arri B mount, sometimes sold with focus gears that have been added. I have seen these sell for around the USD500 mark, but normally people set higher prices on eBay and wait. They do often sell as sets.
Bargains are always possible. Park Rd Post here in NZ, still with a lot of the old National Film Unit gear, are selling off a lot of old stuff right now. They had, maybe still have, a lot of old lenses they were selling. The start bids are low. Selling on eBay and Trademe (NZ). Maybe one can bid from outside NZ.
Park Rd Post on eBay. For example this sweet Cannon zoom, which I think will cover S16 from 8mm, sold for $406. You can use this to link to other gear they sell.
Park Rd Post on Trademe. For example this Cooke Kinetal prime sold for NZ$160, about USD128
I think I just emailed you testerday that I have another (cheaper) TS/PL adapter I am selling ($230). I also have a TS/Arri S adapter for sale ($150)
If one relaxed the sharpness requirement a little, then the old Scneiders are actually quite beautiful (I thought). And maybe one could use standard 16. A nice chance to go fully film through post.