Whatever is available when I need it, usually. My tool collection has been gathered piece by piece as I've needed them over the decades, starting from a set of sockets and wrenches my dad got me when I was 16 (which I still have, BTW). Brands are all over the place. I used to get Craftsman, but they just don't seem to live up to the hype. Branching into power tools, though, I've had my best luck with Dewalt. I've actually got a Dewalt battery powered half inch impact driver that I've been using on the tougher bolts on the truck -- that thing is great!
Edit: On the Craftsman tools, I just meant they don't seem any better or worse than the other brands. I do know I've broken plenty of Craftsman sockets over the years, but I can't say for sure whether any other brand would have fared better.