For Google SEO I would say its semrush.com For Amazon SEO it's sonar-tool.com (you can enter an ASIN and see all the keywords your competitor's product is ranking for and it's free)
There are huge number of tools to see competitors keywords online, like: Google Keyword Planners, SEMrush, Spyfu etc. But these tools never inform a proper result, it may get a normal idea from these tools. So the best way to see competitors keywords manually. Like: you can check allintitle (less than 500 is low competitors), ranking difficulty (less than 40 is low competitors), for your keywords.
From the internet you may find several tools to see the competitors backlinks, PA, DA, keyword difficulty and other necessary stuffs which is necessary in the purpose of SEO. From my personal experience i will suggest you to use moz, adword keyword explorer and semrush