The best transit app I use is this ancient mobile website from my local transit agency. It just has a live map of a chose route and shows where the busses are, and it looks like its some original mobile web app type of stuff like from 15 years ago. What makes it great is how lightweight it is. If I have a spotty data connection or I want to preserve battery, I know I can rely on it a lot more than google maps or other bloated transit apps.