To be honest, I'd always say having some form of intake is the best, it allows cool air to be pulled in and then pushed out by another fan.
The most common setup I've seen is to have some form of intake in the front, so that could be either 1 or 2 fans. The exhaust fan would work with the front intake which takes some of this air and passes it past (or through) the processor fan, giving it a little extra cool air. That intake fan/s on the front also pulls cool air over your hard drive/s, the PSU and the PCI slots, which if using a graphics card which blurts out hot air into your case, funnels out the back through the exhaust or through open PCI ports.
While an exhaust on the top of the case is still pretty good (heat rises, yo), containing it to exhaust out the back is just as good imo.