*be best left to = 委ねる、任せる、

The naysayers argue variously that such matters are best left to the OECD, that an EU-only approach would breach international treaty obligations, and that it could hurt European firms as well as American ones.

Digital taxation - A tech-taxing tangle The Economist Dec. 8th 2018 ハイテク企業に対する課税政策

から