If you could choose each time which subclass features you get - what would you do?

If you could choose from all subclasses of your class each time you get to a level that grants you a subclass feature - what would you play? What subclasses would you pick? Anything that stands out to you?

Assuming you can pick from all books.

Edit: I meant that you can pick each time you would get a subclass feature which subclass of your class for that lvl you can pick. So as a rogue you could pick the 3rd level Arcane Trickster features at lvl 3, the 9th lvl Tief Features at lvl 9 and so on.