The best lens I've used wide-open lately has been the Nikon 50mm F1.2 AIS version - simply phenomenal. I assume a Leica Noctilux is better, but I'll never be able to afford one. The Canon 50mm F1.2 is usable wide open, but nothing to write home about (fine lens overall however). The CV 50/1.1 has a pretty good rep wide open or otherwise.