Steel: Franklin 3, Franklin 8, Wythe, Bedford 8, and Brighton 7. Steel rides smooth and takes a beating. It's the classic city bike material for a reason.
Chromoly steel: Roebling and Lorimer. Chromoly (chromium-molybdenum) is a premium, lighter grade of steel that keeps steel's durability and shock absorption while shedding weight. Both frames are double-butted: thicker tubing at the stress points, tapered in the center, for an even lighter bike.
Aluminum: Willow 8i Disc and Driggs 8i Disc. The Driggs 8i frame is 6061 aluminum. Aluminum saves weight, which matters when your bike lives up a flight of stairs.
None of these is "better." It's a question of ride feel, weight, and how you'll use the bike. Not sure which suits you? Take the Bike Finder Quiz or just ask us.