316L

316L is marine-grade stainless steel, sometimes referred to as 'surgical grade' and the most common in watch cases. [904] is what Rolex uses, but the differences aren't really useful - 904 is better in high-temp sulfuric acid, not common on wristwatches.

Update 11/22/11 WUS just posted a nice long writeup. 316, 904, surface hardening techniques and more. Lovely.