If I had to choose now, I'd probably buy the Qnap TS-509 Pro Turbo. According to the NAS Charts at smallnetbuilder.com, it's about the fastest one out there, only to be topped by it's own (a lot more expensive) TS-809 Pro.

The fun part about the TS-509 Pro is that it uses an Intel Celeron CPU instead of some network CPU which is used mostly in these type of devices. The cool part is that it's also sitting in a CPU socket, so you can actually upgrade/mod the CPU to a Core2Duo if you need to.