Easy answer...the one who respects your money as much as you do. There are 5 BMW dealerships in easy reach of where I live; I've visited them all when I was shopping for BMWs and at various times did business (purchased/leased vehicles) with 3 of them. The reason on those times was that they had the cars with the features I wanted and they respected me. I don't discount word-of-mouth when I'm looking for a "good" dealer, but I also don't lose sight of the fact of my power as a purchaser. In addition, there is likely competition in your favor if there are several BMW dealerships near where you live.
Currently: 2015 Porsche 911 GTS and 2016 Cayenne S eHybrid
Past BMWs: `05 645CiC; `07 650i; `08 M6 convertible; '08 135i; `11 750ix MSport; `15 M235i convertible
Others: '11 Panamera 4S; '13 Boxster S; '13 911 C4
"You think that's air you're breathing now?"