I started on FFN with my original fandom, but I moved over to AO3 in probably...2016 and haven't looked back. I love it because I can download the .epub file and read on my phone/tablet/computer (Apple ecosystem) and it's great for preserving my favorite stories.
Let's start with a simple question: what's your favorite fanfic? If you can't decide, maybe break it down: favorite one-shot, favorite long-form (100k+), etc. There's so many great stories in the fandom!