Not only is it supposed to be the oldest, but it brews some of the best. This is effectivelty the brewery tap and serves the whole range. You have to try both the Vitus (a Weizen Bock) and the Korbinian (a dark Doppelbock) - both wonderful. They also have a well-stocked shop on-site that you can drive right up to. It's worth the walk up for actual drinking though, as you feel like you've earned a couple when you get there (and you wouldn't want to drive afterwards!) Take the footpath that winds steeply uphill from near the crossroads at the bottom of the town. Service was a bit quirky this time from a slightly eccentric waiter, but we were never left waiting for a beer. Having eaten here before I can testify to the food also being excellent.
If you own or manage Weihenstephan, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.