My dream lens is one that is reasonably fast (f/2 at least), has exemplary sharpness right from the maximum aperture, is affordable, and has a camera that delivers great image quality up to ISO 1600. Not much to ask for but alas, there is not such a lens in the market.
Only Leica lenses seem designed to work well right from the maximum aperture. An illustration of this can be found at this comparison of the Nikon Millennium S3's 50/1.4 versus Summilux ASPH 50/1.4. Okay, this is not a fair test, as the Nikkor is a remake of a very old design, but I am pretty sure that the current SLR standard lenses from most brands, including Nikon, do not match the Leica's performance at f/1.4.
But the Leica is way too expensive. And doesn't have a camera that can exploit the quality of the lens.