That's kind of a challenge, as warm weather is typically not that great for long pants. So it needs to be something that vents well, dries fast, and is tough. A tall order.
I might start with Patagonia's Men's Stand-Up Pants ($80, and on sale now for $40). These are made from a very heavy-duty cotton canvas, which has the advantage of being tough and breathable. Not so good at drying fast, but whether that is an issue depends on where you wear these. Theyre great pants. Tough as nails. Just be sure to wash them ten times before you wear them. They can be STIFF.
Arc'teryx's Rampart Pants ($89) also are nice. They're a little different than the Patagonia pantsthey are made from a nylon/Spandex material to add some stretch and toughness. In dry weather, I'd prefer the Patagonia's cotton. If a little damp, these would work better. A bit stylish, too.
For something thats kinda in-between, check out ExOfficio's Gobi Pant ($108). Polyester, nylon, AND cotton. Quite a nice blend, actually, because polyester dries faster than nylon, and cotton adds breathability. And pretty tough. These are very comfortable from the start, yet have good durability. The best of the three? I dunno. I still like the Stand-Ups. But if you are headed for the tropics, check out these.