SEMrush.com / Moz.com /  Ahrefs.com for SEO performance (find out for which keywords a site is ranking) and Backlinks performance. Moz is also used for Domain Authority metrics.
Try BuzzSumo.com for Social Media Performance metrics.
SimilarWeb.com offers data beyond search engine traffic, e.g. how much of the traffic comes from direct or referral sources.
Use Google Analytics and Google Search Console for comparing data from external tools with client domain data from internal web analytics tools.