AAA, Costco, etc. are all nice for having pre-negotiated prices, but IMO the best way to buy a car is something like CarWoo -- that way you might actually get a "deal" if you pick the right make/model, vs. a global price. I'd use them for my next car except I plan to either get a used diesel truck or a Tesla, and they only do new cars (and probably not Teslas).

(Although the absolute best deals I've found on cars have been deployed military BMW/Audi sales programs)