On my M135i, I've gone for these 18" 385-style winter alloys (with Pirelli Sotto Zero runflats) which are one of the official BMW sets. More expensive than the 17" options but I think worth the extra money - I think it looks underwheeled on 17"s. Have to say I was not totally sold on the grey wheels when I first got the car but now, having seen the silver finish, I find myself preferring the grey finish:
Another one with my other black BMW!:
__________________
____________________
Current: Macan GTS
Previous: 440i GC, M135i, E60 M5, Z1, 330d, 135i, Alfa 156 GTA, Mercedes 500E, Audi S3, Audi A3 quattro, VW Golf GTI 16v