For most urban planter boxes in a rainy climate, the best all-around material is UV-stabilized high-density polyethylene, or HDPE. Commercial-grade HDPE does not absorb water, rot, rust, or swell like wood, and it will not corrode like untreated steel. With proper UV inhibitors, it also resists fading and cracking from sun exposure between rainstorms. It is lightweight, easy to clean, and works well for both residential courtyards and public streetscapes.
If you want a more premium architectural look, powder-coated marine-grade aluminum is an excellent second choice. It is strong, lightweight, and naturally corrosion-resistant, and the powder coating adds a durable color finish. Avoid bare mild steel and untreated wood, because both will degrade quickly in constant moisture. Galvanized steel can work, but cuts and scratches need repair to prevent rust. Concrete is heavy and can crack in freeze-thaw cycles, while low-quality composites may fade or become brittle over time.
The material matters, but the details matter just as much. Choose a planter with raised feet, generous drainage holes, and a slight slope toward the outlet. Use a coarse drainage layer or a geotextile filter to keep soil from clogging the holes, and consider a removable inner liner for easier maintenance. In very wet climates, keep the planter off standing water and check drainage seasonally. For most rainy urban settings, UV-stabilized HDPE gives the best balance of weather resistance, low maintenance, and long service life, while powder-coated aluminum is the best upgrade for a sharper design finish.