If you are furnishing a balcony, rooftop, courtyard, sidewalk seating area, or shared urban patio, the first thing I would do is separate materials that truly do not rust from materials that are merely rust-resistant. That difference matters a lot in cities, where rain, pollution, salt, and regular cleaning can wear down cheap finishes fast.
Powder-coated aluminum is one of the best all-around choices. Aluminum itself does not rust like steel; it forms a protective oxide layer, and a good powder coating adds color and extra protection. It is lightweight, easy to move, and works well for chairs, tables, and benches. Look for thick powder coating, sealed welds, and stainless or aluminum hardware. Avoid very thin painted finishes that chip easily, especially on coastal streets or windy rooftops.
Marine-grade stainless steel is another strong option, particularly for public spaces, benches, and frames that need extra durability. Grade 316 stainless steel is better than 304 in salty or polluted environments. It will not rust like ordinary steel, but it can show tea staining or surface discoloration if neglected, so choose a brushed or easy-clean finish and wipe it down occasionally.
Recycled HDPE, often called poly lumber, is a practical rust-proof choice for urban outdoor furniture. It does not rust, splinter, or rot, and it handles rain and sun well. It can be heavy and may fade slightly over time, but good quality HDPE is excellent for benches, picnic tables, and planters where low maintenance matters more than a metal look.
Teak and other dense woods are not rust-proof because they contain no steel, but their hardware still matters. Teak ages gracefully outdoors and needs little more than occasional cleaning; oiling is optional depending on the look you want. Make sure any screws, brackets, or connecting plates are stainless steel, or the wood will outlast the fittings.
Concrete, stone, and composite stone are also rust-free, making them good for tables, benches, and planter surrounds. They are heavy and stable, which is useful in busy urban areas, but they need sealing in freeze-thaw climates and can be difficult to move.
For metal furniture, avoid mild steel or cheap wrought-iron-style pieces unless they are hot-dip galvanized and properly coated. Galvanized steel can be very durable, but it is rust-resistant rather than rust-proof, and any chip or scratch should be touched up. Also check the small parts: stainless steel bolts, nylon glides, drainage holes, and replaceable feet make a big difference in real urban conditions.
My short answer: choose powder-coated aluminum for lightweight versatility, 316 stainless steel for heavy-duty public seating, HDPE for the lowest maintenance, and teak or concrete where looks and weight suit the space. Whatever you choose, buy the best finish and hardware you can afford, and avoid furniture where the frame is protected but the fasteners are not.