It probably really depends on the project, though I’d probably try and start with the tests that are easiest/nicest to write and those which will be most useful. Look for complex logic that is also quite self-contained.
That will probably help to convince others of the value of tests if they aren’t onboard already.
Well I kept using it until Infinity died, which was only at the start of this month!
If I do decide to go back, it will be by compiling the infinity APK with my own API key, but I’m not feeling much of an urge to bother at the moment.