Oh, hold on, I haven't participated in this thread yet? A travesty. When I was in middle school, my whole class (well, I exaggerate, mostly boys, but still) was obsessed with this mobile game called Gravity Defied. It's basically this very simplistic game where you're driving on a bike controlling its speed and balance, trying to complete all the different levels and not wipe out. There's multiple difficulty levels and for completing each one you get a more powerful bike (and suddenly having more horsepower entirely screws with your balance strategy lol).
Now, the thing that made it infinitely replayable was the existence of a free level creator. The amount of versions and mods out there was outstanding. And some of them were downright unbeatable, because, like, there weren't any limits or guidelines, you just drew those lines however you wanted and the only way to check if that obstacle was even passable was to launch the game on your phone and try to beat it a 100 times or so.
I distinctly remember making my own version, too. Proud to say that it was, in fact, beatable, as tested by me and my classmates whom I passed the file to. Not proud to say that, if I remember correctly, the color scheme was eye-bleeding-bright, as is the custom when you're a creative teenager in the 00s. Sad to add that there's no way to recover that file now. A pity, I'd like to see it. Betcha the level names were also highly inappropriate ('cause that's cool, right)!
The reason I can confidently say that this game is my favorite mobile game is that, well, I still play it. The thing about Gravity Defied is, touch screens kind of killed it. I know there are games with similar premise today, and they are played on smartphones well enough, but I was way too used to playing it on buttons. Good old physical phone keyboard. Never managed to make the switch.
At some point I did manage to find an Android version of the most classic build that someone ported WITH the emulated buttons. The creativity of everyone in the community making their own builds is lost, however.
That being said, I did get myself a fully functioning replica of Sony Ericsson W810i a while back. And the first thing I did upon getting it to work was to install the original GF. Thinking about making my own build again, just for fun - the level creator is still available online all these years later!