Top 7 Tools to Check Website Traffic of Your Competitors:
1. Compete.com
2. Google AdWords Display Ad Planner
3. SEMrush
4. Alexa
5. Google Insights
6. Ahrefs.com
7. Intention to Advertise
Alexa:Alexa offers one of the best website traffic checker tools. Moreover, Alexa is quite popular with webmasters and bloggers as it enables you to get insight into the website rankings. this tool is best for checking website traffic.
If you are referring to your own website, then is Google Analytics. For other's websites, Similarweb.com is a free tool to check (sometimes is not accurate though). Paid one you can try compete.com or semrush.