As many of you will know, I like doing empirical testing. I have an ISO 12233-based chart that costs more than some L-series lenses. I use that expensive chart to test new lenses for flatness of field, centering defects, etc. But, my definition of tack-sharp is both simple and practical. I take a portrait-type shot of one of my daughters, with the lens wide open and the focus on one of her eyes, and if I can count each and every one of her eyelashes, I consider the lens to be sharp.
So only when wide open? What camera - ff or crop?
FF would rule out several lens that if tested on a crop would be considered tack sharp - especially the 1.2 lens