My vote goes to Dell laptops. Even though I hate them with a passion, having worked as a Dell service technician, I have to admit that they have probably the best warranty plan out of all the major manufacturers (on-site tech, cross-shipping, you name it, they've got it all). Which is all you are buying, really, because your laptop WILL have problems, one day or another.
I would suggest Latitudes because that's their "standard" business models which every Dell Tech should know how to fix (You'd be surprised how many techs make the problems worse on XPS and Precisions...), but from what I've heard they are significantly more expensive. I would also suggest purchasing through your company (if that's an option) as that makes dealing with Dell CSR much less of a hassle.