If I had to choose the ONE tracking tool, which I could have in every analytics setup, it would be Google Tag Manager (GTM). It's the Swiss Army knife of tracking and can do so much without having to code. Need to track how many visitors click a button, submit a form, or scroll down at least 50% of your website? Good luck doing that with Google Analytics or the Facebook Pixel without adding custom code to your website. With Google Tag Manager, you have to install it once, and you can do 99% of the work from the browser interface.
GTM enables marketers and product people to become more data-driven without dependence on the dev team. It frees you to define the tracking events that help you understand your customer's behavior and measure the performance of your campaigns.
I want this to be the one short resource you can go to whenever questions come up like:
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