Simple.
N or C camp--
If you're on a budget, get a 430EXII or SB600 (or SB700- rumors?)
If you're not on a budget, and would like to get the best of the best, get a 580EXII or SB900 (or 800 if you can find one used for a good price)
For the purchase of your first flash, the only things that matter are your budget, the power you need, and how often you will shoot with the flash. Anything else like master/slave, stroboscopic, built yadayada should come in second to the priorities above.