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.
At least the last season of scrubs was a different enough setting/cast, so even though it was definitely not as good, it didn’t “ruin” it for me like some other series that have gone on too long.