Oh we all love our Panasonic 20mm f/1.7
On Ebay we just found the extream rare De Oude Delft 50mm f/0.75 lens (click here)
But guys that’s a 50mm f/0.75!
If you want it faster you need to find the Carl Zeiss 50mm f/0.7. It is a Limited production lens built for the NASA space program- Stanley Kubrick used it on a 35mm movie cameras for some candlelit scenes in Barry Lyndon! Story of the Zeiss lens is available here: http://www.visual-memory.co.uk/sk/ac/len/page1.htm