If I had to pick one material for the structural frame, I’d choose marine-grade 316 stainless steel. In a rainy city, 304 stainless can develop tea-staining around welds and fixings, especially where road salt, pollution, or standing water are present. Grade 316 has molybdenum, which gives it better pitting and crevice corrosion resistance. It also holds up to repeated cleaning, which matters for public furniture. The catch is that stainless steel is not magic: use proper welds, passivate the surface, avoid trapped water, and specify non-marking, drained feet.
For seating surfaces, I’d pair it with HDPE slats. HDPE doesn’t rot, swell, splinter, or need annual oiling, and it handles wet-dry cycles far better than most timber in heavy public use. Teak and ipe are beautiful and durable, but they need maintenance and can become slippery or greasy in constant rain. Powder-coated aluminum is a good budget option if the coating is thick and the frame is well drained, but chips around fixings can start corrosion. Concrete is tough but heavy and cold. So my practical answer is a 316 stainless steel frame with HDPE slats and drainage-focused detailing. That combination is the most reliable long-term choice for urban outdoor furniture in a rainy climate.