For the front anyway, I run a nice generic 4" speaker. You need to modify the mounting but it's not difficult. Just drill a few holes in the door card and held in with screws from hardware store.
What you need to be careful for is depth and protrusion of the speaker. I run these specifically. Personally, I would choose whatever 3.5/4" you want, ensure it fits, and run that vs bav sound. You may be stuck with one of their amps though, few fit the bmws.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Running those speakers and they work really well for my system. Simple, not overpowered