If I had to pick the best outdoor bench wood for a rainy climate, it would be teak—no contest. Teak is naturally oily and dense, which makes it a highly rot-resistant wood. It resists moisture, rot, and fungal decay, and it stays stable through repeated wet-dry cycles, so it warps and cracks far less than most woods. The real downside is price. A genuine teak outdoor bench is expensive, and cheap look-alikes are not the same.
If teak is not in your budget, ipe is the next heavy hitter. Ipe is so dense that water barely penetrates it, and it is incredibly rot-resistant. The trade-off is that ipe is heavy, hard to cut, and usually needs pre-drilling. White oak is a more affordable domestic option and an underrated weather-resistant wood. Its pores are naturally clogged with tyloses, which block water movement, which is why it has been used for barrels and shipbuilding. Quarter-sawn white oak is especially stable. Cedar, especially western red cedar, is lightweight and naturally decay-resistant, but it is softer and will show dents, mildew, or moss faster in constant dampness.
Avoid untreated pine, poplar, birch, beech, and most generic softwoods for a bench that will sit in rain. They absorb water, swell, cup, and rot quickly. Pressure-treated pine can work, but it often warps as it dries and is not in the same league as teak or white oak.
Wood choice is only half the battle. A slatted seat with drainage gaps, sealed end grain, stainless steel or coated deck screws, and keeping the legs off constantly wet ground will make any bench last much longer. Even the best wood will fail early if water is trapped in a joint.
So my honest ranking for a rainy-climate bench is: teak first for low-maintenance decades and stability; ipe second for extreme toughness; white oak third for a great balance of rot resistance and cost; cedar fourth for light-duty budget builds. If you want the closest thing to a buy-it-and-forget-it bench in heavy rain, teak is still the answer.