Gravity Falls has that first factor that makes it feel so special. If I had to rank them all, GF would be first, followed by Amphibia and then TOH.
Gravity Falls was the first big cartoon I watched after Phineas and Ferb in 2015 (minus Milo Murphy's Law and Star Wars Rebels) and the one that got me hooked for being so unique and special in ways I still do not fully understand how. I think the combination of a show in the Pacific Northwest, a rarely explored region in cartoons, the characters, mystery and more was what helped. Amphibia was a close second, but Owlphibia never to me captured the same love I had for GF.
I see it as a lightning in a bottle at the right place at the right time. When I got into Gravity Falls, I was at a low point in my life where I felt like I didn't know what to do or where I was going. It gave me a sense of new purpose and passion and a deviation away from those issues. Nowadays I get more I was feeling like Mabel in Vs the Future during those times and GF really was what helped me out.
It got me through tough days, got me into a new hobby and passion, re-unlocked my love of video editing and strive to keep growing with it and more.
I am who I am today cause of Gravity Falls. Similarly, there's probably someone else out there who feels the same about Amphibia or The Owl House. It was a show that was in the right place at the right time for them and helped them get through a tough time.
It's not so much therefore a question of if Gravity Falls is better...but how much it holds itself near and dear to you? We all have a passion we love dearly and care a lot about. For me, that's Gravity Falls and aviation. For someone else it could be Amphibia and knitting or The Owl House and reading.
These shows mean something to us in our own ways. And what that means is what matters above all else.