What tent will stay dry in torrential conditions?

A: Honestly, I'm a little reluctant to believe that these tents are "failing" in the sense that the fly is actually letting water through it. Tent flies are almost always made of nylon, usually taffeta or ripstop, with a polyurethane coating. This creates an impermeable membrane. Both The North Face and REI also pressure-test this fabric to ensure that even water under moderate force cannot work its way through the fly. Rather, I think that condensation is simply forming under the tent fly. That would certainly create the impression that the tent fabric, not the seams, is your culprit for the leak; it would also explain why additional sealing has had no effect. Rain tends to exacerbate condensation by raising the humidity and more radically cooling the fly material in comparison to the warm, moist air rising out of the tent. There's no real solution, because in fact the tents are designed to do this. The idea is that moist air leaves the tent, condenses against the fly, then is carried to the ground by the slope of the rain fly.

That said, it's also true that these coatings can wear off if the tent is, for example, subjected to heavy use or pounded by dust or mud. It's not likely, but it's possible. While I don't want to slander either The North Face or REI, both of whom make fine products, I would offer the opinion that some TNF tents, and most REI ones, skew slightly toward the "light construction" end of things. Moss Tents, now sold under the MSR label, were long regarded as some of the most heavily built in the business, a reputation they still hold. Take a look at the Phantom ($499), a big, four-season tent that's built for heavy use in just about all conditions. You might also consider a Bibler single-wall tent, such as the Ahwahnee ($699). Bibler tents use a proprietary wall material that does without the fly. Water vapor clings to its inside surface briefly, then transpires through the tent wall. They sleep very dryly.

For any tent, though, excellent ventilation is the key to keeping the inside dry. Set the tent up so any breezes can blow through its length, flushing out moist air. And adjust the fly so there's a gap between it and the ground around its perimeter, which lets air circulate in. If the fly has any hooded vents higher up, make sure they're open. A little rain coming in will actually result in less moisture than condensation.

Additional resources: REI; The North Face; Mountain Safety Research; Bibler.