You can certainly ask other people's opinions about such things, however maximum sharpness is a function of every component in an imaging system, including the sensor. For someone using, say, a Canon 1D IV or 1D X, where the maximum theoretical sensor spatial resolution is less than 100 lp/mm, their best aperture is likely to be higher than someone using say the Canon 7D, which has a maximum theoretical sensor spatial resolution of 116 lp/mm, and could realize better resolution at wider apertures.
Theoretically speaking, most lenses will peak in sharpness at around f/4-f/6, depending on the specific characteristics of the lens. Top-grade professional lenses, such the more expensive Canon L-series lenses, can potentially peak near their maximum aperture, with the top few supertelephoto L-series lenses released in the last few years doing just that (the 300mm, 400mm, 500mm, and 600mm Mark II L-series lenses are all nearly perfect, and most achieve maximum sharpness at their widest aperture, or within 1/3 to 2/3rds of a stop from it.)
If you want to figure out the "sweet spot" for any lens, you can use
Reikan FoCal Pro (
http://www.reikan.co.uk/focal/focal-pro.html) to do so. It has an Aperture Sharpness tool that takes photos at every aperture, from the narrowest to the widest, and evaluates IQ for each one. I recently ran an evaluation of aperture sharpness for my copy of the
100mm f/2.8 USM Macro lens (after using FoCal Pro to do what I purchased it for...automatically tune AF micro focus adjustment for maximum performance). My test results, after several runs, indicate that f/4.5 is the optimal aperture for this lens (at least with the 7D's 18mp sensor). This should remain true with larger sensors that have lower spatial resolutions as well, such as the 1D X or 5D III.
