You will see a lot of opinions regarding the best with wheel only prices ranging from about $1000 to $4000 for a set of 4. Wheel and tire replacement and upgrade can really be an enormous undertaking with so many options for size, fitment, color, design, performance level, etc. If you have the ability to install a set of wheels with mounted/balanced tires on your own, Tire Rack is a great option. They can also set you up with approved installers that will charge about $100 to put them on. If you have a good tire shop near you such as Discount Tire (good in my opinion), you could order separate wheels and the tire shop can order in any tire.
I would look at 19 or 20" wheels and check out VMR, Hartmann, Avant Garde for less expense, and BBS, OZ, Vossen, Forgestar for some pricier options. I think the best wheel spec for an average Q5 user is 20x8.5, offset ET25.
Tires are more important and more specific to your driving habits, environment, and uses of the vehicle. 255/45/20 fits the wheel spec I mentioned above. I like the new(ish) offerings from Michelin in the Pilot Sport A/S 3 and Pilot Super Sport, but those aren't a perfect match for everyone.