For me it was a couple reasons:
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my brother installed Ubuntu 12.04 on my desktop for me when I was in high school, and I was enamored with the different desktop layout. It got me started on the journey.
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maintaining it is much easier than windows. Running one command/script to update a system is much faster than heading to the right window or menu and hoping Microsoft delivers you an update. Plus if it breaks it's easier IMO to troubleshoot and fix.
I'd go with either Firefox or Thunderbird. Both are immensely useful pieces of software that I use on a daily basis, and have evolved (mostly) nicely over time.
Not to give Mozilla too much credit, Nextcloud is also pretty slick!