Though Ag nanocrystals are predicted to be the best material for localized surface plasmon resonances (LSPR) in the visible light region, realization of their ideal LSPR properties is hindered by the stringent requirements: i.e., simultaneous control of their size, shape, crystallinity, and surface structure. To achieve this goal, a synthetic scheme in non-polar solvent coupled with mild oxidative etching by H+ ions has been established. With a trace amount of Cl- ions as catalysts, H+ ions (in the form of carboxylic acids) become active for selectively etching the nuclei (and small nanocrystals) with imperfect crystal structure, which results in a new growth mechanism for formation of monodisperse nanocrystals, namely "self-focusing of size/crystallinity distribution". H+ ions, ligands, and other reagents in the scheme are confirmed to possess negligible effects on the surface dielectric properties of Ag nanocrystals. To eliminate radiative damping of LSPR, single-crystalline and monodisperse spherical Ag nanocrystals in the size range between 7 and 20 nm are synthesized using this one-pot scheme. With excellent control of all related structural parameters, the full width at half-maximum of LSPR spectra of single-crystalline Ag nanocrystals match theoretical predictions in the entire size range, and the maximum quality factor (similar to 20) of LSPR predicted by theory is realized. The Raman enhancement factor of the single-crystalline Ag nanocrystals for crystal violet (excitation at 514 nm) is 5 times higher than that of the typical multi-twinned nanocrystals with the same size.