Which is the best tool for check traffic?

For competitor sites, the numbers will always be estimates you can get close, but never the exact traffic

If you’re checking competitor traffic, there is no tool that gives 100% accurate numbers because third-party tools use estimates. Similarweb is one of the better options for estimating total traffic and traffic sources, while Ahrefs is better for estimating organic search traffic. For your own website, Google Analytics and Google Search Console are the best because they use your actual data. Similarweb has a free option, while advanced features are paid.

no third-party seo tool can show 100% accurate traffic, since most are just estimates.