The big Belgian beers age well -- Belgian Golden Strong and Dark Strong as well as the Trappist Dubbel and Tripel.
Old Ales, Doppelbocks, and Imperial Stouts also age well. I think you'll find that most high gravity beers age well. The only exception to this might be Imperial IPAs which don't age so well because hop flavors & aroma are too diminished with age.
Currently On Draft: Bamberger Rauch Dunkel, Belgian Blond, Pilsener Urquell clone, Smoked Porter
Bottled: Concord Pyment, Mi'Apa Sparkling Mead, Chimay Blue, Old Simcoe American Barleywine, Old Cantankerous
Fermenting and Conditioning: Pseudo-Decoction Munich Dunkel, Left Hook Bitter
Recently Kicked Kegs: Fresh Hop Pale Ale, Citra Rye IPA
On Deck: Old Rasputin, Northstar IPA, Ur-bock Dunkel