For most urban planter boxes in a rainy climate, the best material is high-density polyethylene, or HDPE. It does not rot, absorb water, or rust, and when it is UV-stabilized it can handle sun exposure as well as constant moisture. Choose a thick-walled HDPE planter with integrated drainage, overflow outlets, and small feet that keep the base above the pavement so water can escape and air can circulate.
Powder-coated aluminum is an excellent second choice when you want a more architectural look. It is lightweight, strong, and corrosion-resistant if the coating is properly applied and cut edges are sealed. Stainless steel is also very durable in wet conditions, especially marine-grade stainless in coastal areas, but it costs more and may show scratches. Concrete is stable and heavy but needs sealing and reliable drainage to prevent waterlogging and staining. Avoid untreated mild steel, and use wood only if it is rot-resistant, lined, and protected with a suitable finish.
In practice, the best urban planter box for a rainy climate is a well-drained HDPE or powder-coated aluminum system. The material matters, but drainage details matter just as much: drainage holes, a free-draining growing mix, a geotextile filter, overflow outlets, and a gap between the planter and the ground so excess water can move away quickly.