Basically you want a flash with manual controls. Once you have that, getting a PC sync port on it is a nice bonus, if you don't want to go through the hotshoe-to-PC adapter.

Personally I've always used Canon/Nikon speedlites (430EZ, 430EXII, SB600, SB25, SB24 to be precise) simply because I had/have them since I use their 35mm/digital systems. They all worked fine with/without umbrellas or other modifiers and I even used those cheapo wireless triggers from ebay. Just to make it clear, these setups I used with my current Hasselblad kit and I also used them with a Bronica SQB and ETRS, when I owned these two in the past.