With a Bessa-R (or any rangefinder for that matter) I would foremost look for a flash that balances well on top. Although I do have a large Metz flash with all the features that you can think of (tilt up/down, swivel, power), it doesn't sit well on an RF. The thing always hangs upside down on the neck strap. I much prefer a Konica HX18 (no tilt/swivel, moderate power), which sits entirely above the body, and so doesn't topple it over.
The HX18 by the way, was designed to go with the Hexar RF.