In my case, the one tool that i check most often is definitely google search console since it tells me what is really going on after publishing. Planning can be done using keyword research tools, but only google search Console will show you real searches, lost opportunities, and pages to fix.
I cannot work without Ahrefs, so that's one tool I couldn't do without.
If forced to name only one, i would select google search console because it grounds me in reality. It is possible for many other tools to offer keyword and opportiunity suggestions, but GSC tells you how ggl sees your content. It is common for me to uncover some important insights when investigating queries on which a page is showing impressions even though the page has not been optimised properly yet.
My tool is likely a basic note taking app and not an SEO one. Every time i have an interesting question in forums, comments, or random browsing, I write it down right away. In a few weeks, these notes usually make for gud blog posts than anything suggested by my keyword tool.
For me, it is the wayback machine actually. looking at how earlier versions of high ranking pages developed has given me more ideas for content than many keyword tools have.
