In terms of image quality, both manufacturers produce excellent lenses. Canon's top-tier optics are its L-series lenses, which have the iconic red ring around the end of the lens. Many people consider Sigma's Art line, generally possessing faster apertures, to be the most desirable.
For build quality, again, Canon's L-series of lenses are professional quality – they possess weather sealing and are of robust construction. Sigma also offers weather-sealed lenses, so the "best" lenses are the ones that suit your budget and shooting requirements.