Which of these is the most profitable?
Here is a quick comparison of various offerings that DiabloD3 whipped up. You may wish to do some research on each device before potentially getting stuck with something you don't want or like for some reason;
https://bitcointalk.org/index.php?topic=77469.msg887606#msg887606BFL's mini-rig looks to be the most profitable and efficient of the FPGA options, but hasn't shipped yet. The X6500 is the most efficient, but is a bit expensive per mhash/s so has a longer payback period as well as limited availability. If you get in on the cheap pre-order price, the Enterpoint quads are a good bet, and they should ship within a few weeks, but don't have final software support (yet). The BFL Single is the least efficient outside of GPU mining, but also relatively cheap and fast.
Various ASIC based solutions have not shipped yet, and command a premium because of their novelty and efficiency. Reference Largecoin, and possibly SHAlab.