To accurately track traffic you need code on their site (JS is best) - there's no way I know about to accurately track anyone elses traffic.

semrush will share keywords ranking for a website but I think that wouldn`t be all you werte looking for..

Spyfy is def. The best tool .

You can use different tools like The Social Site Explorer. This tool is easy to use. Simply type in your website URL, or your competitors’ website address, and get stats on how many shares your content gets.

alexa is one of the best sites where you can track both your site and the competition, in comparison

My favorite is Alexa. The free version is good enough for researching traffic for competitors.
1)It reports traffic quality in terms of bounce rate, daily pageviews per visitor and daily time on site.
2) It reports audience demographics in terms of gender, education and visitors by country
3) Upstream sites shows the traffic sources.
The traffic ranks are useful only when comparing sites in the same industry. The other limitation is that it doesn't have sufficient data for sites below one million traffic rank.

Some other tools are also helpful for competitor research: SpyFu, Compete, SEMrush, and opensiteexplorer.