What flooring never goes out of style?

Natural oak in a mid-brown tone, and porcelain or natural stone tile. Both have survived several decades of trend cycles because they are natural materials in restrained colours rather than a manufactured look. Solid oak carries a second advantage: it can be sanded and refinished several times, so the same floor can be recoloured to suit the decade instead of replaced. Simple, wide, matte, mid-tone and low-contrast is the safe end of the spectrum.