If you want the ultimate, it's got to be Gore-tex... but look to pay a premium for that.
Aside from that, I'd say at least 15,000 with fully taped seams and that should get you through most days. I'd also be weary of what brand you get and even compare pants within the same brand. I had 25k rated dub pants and i'm not sure what fabric they used for waterproofing but it soaked easily after a couple months. If it's laminated rather than coated on too that's a plus.