Buick once characterized its creations as "the doctor's car," but that was a long time ago. Today's doctors are driving either a Mercedes-Benz or a high-end sport-utility vehicle, and when they're driving them you can bet they're not on their way to make a house call. Nevertheless, the concept behind that slogan--subdued luxury, comfort,excellent road manners and all-weather reliability--applies to Buick's new Park Avenues, the basic Park and the Ultra. It doesn't command the kind of upper middle class cachet it did in the good old days, but for folks moving up from mid-size to full-size--folks who don't feel a need to fortify their status with something from BMW, Lexus or Mercedes--a Park Avenue is worth a serious look. This is a pleasant step up into quiet, roomy luxury, bolstered by high quality and a much more authoritative set of dynamic capabilities.