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.
That video was the first thing I thought of :)
(https://youtu.be/YUpST_cQ1hM for anyone wondering)