If you will be the acting Admin then I'd say all of them except those for integrations that you do not have. Not trying to be flippant, but when an issue arises, know where to go immediately will save you considerable angst.
Question becomes which do you need to know first, and I think alot of that depends on where your team is in their deployment.
If you are early on, then users and roles is important, you will want to be very familiar with Channels and tags.
Field Management is key!