Acura, Infiniti, Cadillac, and Lexus have their work cut out for them, because the current contenders for best mid-size luxury car are all very compelling. They also all happen to be German. Audi, BMW, and Mercedes-Benz rule this sector, if not in terms of overall sales, then at least in terms of perception. Executives and engineers from the other makers consider the A6, the 5-series, and the E-class benchmark sedans. But which of these three is the best?

To find out, we took the six-cylinder models on a lightning tour of Bavaria. They're the volume sellers, after all, even if they lack the pace and ultimate refinement of their pricier V-8-engined siblings. The newest car, the Audi A6 3.2 Quattro, is the biggest. It's roughly three inches longer than its rivals and is less than five inches shorter than Audi's flagship A8. But it fails to carve out a significant space advantage, because its exterior dimensions are offset by the shortest wheelbase in this group. As a result, Ingolstadt's midliner provides a little less rear headroom and shoulder room than the other two contenders. The one area where the extra inches pay off is concealed by a deep and wide trunk lid. In suitcase swallowing, the Audi's trunk narrowly outpackages those of the Mercedes and the BMW.

In the premium segment, styling is an increasingly important decider. The E-class scores well here, with a balanced and elegant, if traditional, design. The 5-series tries hard to look stylish, fashionable, and modern. And the A6 combines coupe overtones with a bold grille and a middle-of-the-road rear end. Inside, the visual and functional differentiation is more obvious. The Mercedes is a downscaled but no less opulent variation of the successful S-class theme. The BMW is an ergonomic challenge, utilizing different architectures and materials, which renders it a marginally tighter fit than its challengers. The Audi is airy and contemporary, even more beautifully put together than the Mercedes, and particularly easy to use.

That sentiment certainly applies to Audi's Multi Media Interface system, which is logical and only as complex as you want it to be. The center controller is flanked by four buttons that are directly related to the four major menus displayed by the color monitor. There are also eight access buttons on the center console, which hook you up with the CD player, the navigation system, or the car phone. The drawback is that simple tasks such as changing the radio frequency or cranking up the seat heater require more than one push, click, or scroll. And the buttons on the steering wheel are underwhelming in the variety of functions they are able to perform. The E320 we tested was unfortunately not fitted with the latest-generation Comand system, which is much better laid out but more difficult to access because of its tiny, multidirectional, multitask controller. Although BMW's iDrive is slowly shedding the idiosyncrasies that haunted the original version in the 7-series, the structure of the software is still more function-key than point-and-click, and simple exercises such as selecting split-screen navigation display will have you cursing and moaning.

While most of the extras on these cars' options lists are aimed at improving comfort and convenience, the BMW 5-series actually needs Active Steering (AS) and Active Roll Stabilization (ARS) to evolve from a more generic sedan into something special. Part of the $3300 Sport package, the variable-ratio steering and the self-adjusting antiroll bars give the 530i an advantage in handling and roadholding. While Active Steering is go-kart quick in town or on twisty mountain roads, it feels nicely balanced and relaxed on the highway. With Active Steering, the BMW reaches a new level of feedback and precision, but it takes a while to adjust your driving style to it.

You don't have to pay extra for the unique selling propositions fitted to the Audi. Quattro four-wheel drive, for example, is standard, direct fuel injection comes free of charge, and there's a push-button electromechanical parking brake. Powered by a new 24-valve V-6 that delivers 252 horsepower at 6500 rpm and 243 pound-feet of torque at 3250 rpm, the Audi can accelerate from 0 to 62 mph in 7.1 seconds, which is on par with the 225-horsepower 530i and the 221-horsepower E320; they take 6.9 seconds and 7.1 seconds, respectively, to sprint from 0 to 60 mph. In the United States, the BMW runs out of steam at 150 mph, but the Mercedes is limited to 130 mph. (In Germany, the Audi is governed at a top speed of 155 mph.) The more powerful and heavier Audi is also the thirstiest: while the A6 averaged just under 17 mpg on our test, the E320 returned more than 17 mpg, and the 530i was the most frugal, at just over 18 mpg.