Honestly, there isn't one magic material that wins everywhere. The most durable bus stop or transit shelter furniture is usually a well-engineered combination: a heavy-duty steel frame, a tough coating, and a seat surface that can take weather, graffiti, and constant use. If I had to pick a default, I'd choose hot-dip galvanized steel with a thick powder coat or thermoplastic coating. The galvanizing protects the steel from rust even if the coating gets scratched; the outer coating handles UV, rain, and everyday abrasion. Add stainless steel fasteners, ideally 304 or 316 in coastal areas, and you've got a setup that lasts for decades with minimal maintenance.
For the seat itself, solid HDPE, or high-density polyethylene, or a reinforced composite is very practical. It won't rot, splinter, or absorb water like wood, and it's resistant to many graffiti paints and cleaning chemicals. It can fade over time, but UV-stabilized grades hold up well. If you want a more premium, nearly indestructible feel, aluminum or stainless steel seating with a powder-coated finish works, though metal can get hot in direct sun and cold in winter unless it's perforated or coated.
Concrete and cast stone are also extremely durable for bases, legs, and entire benches. They're heavy, so they resist theft and vandalism, and they handle freeze-thaw cycles if properly air-entrained and sealed. The downside is weight: you need solid footings, and installation is more expensive. Concrete can also chip at edges, and it's not comfortable without a wood, composite, or HDPE top.
What about wood? Ipe, teak, or black locust can last a long time, but they need maintenance, can be carved or burned, and will weather. For a busy transit shelter, I'd use wood only as an accent slat over a steel frame, not as the primary structure.
If vandalism and graffiti are major issues, stainless steel 316 with a brushed finish and concrete bases is probably the most bulletproof combination. It's expensive, but it cleans up well and resists corrosion. For coastal cities, 316 stainless is worth the upgrade. In inland areas with moderate budgets, galvanized steel plus thermoplastic coating plus HDPE seats is the sweet spot.
So the most durable choice is usually a hot-dip galvanized steel or 316 stainless steel frame, stainless fasteners, a concrete or cast stone base, and HDPE, aluminum, or stainless steel seating. Avoid thin tubular steel, untreated wood, and cheap plastic. Specify commercial-grade hardware, tamper-resistant fasteners, and replaceable components. That way the shelter furniture can survive years of sun, rain, salt, graffiti, and rush-hour crowds.