For a GT3 (this does not apply to any of the other 911 variants) without a trace of doubt: white with black painted wheels (leaving the outside border or rim of the wheel unpainted).
Seeming to deviate a bit here, but I think that Meteor Metallic Grey looks great on the magazine scans and videos that I have seen. However, I'm personally not sure if I would be brave enough to plump for it if I were to be ordering a GT3, having never seen it in real life.
Loki, that car is beautiful. Do you have any more pictures of it?