Sockets and Ratchets.
BMWs love Torx fasteners, so a Torx Socket Set and E-Torx Socket Set are must-haves. Add a full metric socket set in 1/4, 3/8, and 1/2 Drive, and you'll cover almost every job — from interior trim to suspension work.
For ratchets, don't cheap out. A good 3/8 or 1/4 ratchet with plenty of teeth makes life so much easier in tight engine bays and under the car. Less swing per click, smoother operation — it's worth it. I used to use the cheapest ones from HF. I would never go back after upgrading.