Congrats, you've been dating for a full year. Now, you know she's going to expect some over-the-top shit and your full dedication for celebrating this occasion. If you take her down the street to your favorite hole-in-the-wall restaurant and dress like a chill cozy boy, you might not make it any further past the one year mark.
Instead, you're gong to need to wear your "nice" stuff that she knows you don't break out that often. A suit is completely acceptable in this situation, but don't look like a peacock trying to stunt at Pitti Uomo. Wear something slim, and dress the look down a bit by going with a knit tie. It's the perfect accoutrement to your thought-out ensemble. Also, don't forget the flowers.