It depends. I use Metz for all my cameras. Some of the Metz flashes have replaceable electronics and shoe (SCA in Metz terminology) which is designed for each camera brand, so when I move from Olympus to Nikon and back, I change the SCA. Another reason why I use Metz, is that they have the "potato masher" style flashes with much higher output than conventional flashes (70 or above), which also makes a practical grip next to the camera. I've never liked the flash towering above my camera, and I've broken the shoe off a couple of them.
Metz is probably the best third party supplier of flashes and very reputable. Their models aren't cheaper than corresponding Olympus or Pentax flashes, but if you use more than one camera system, it may be worth a look:
http://www.metz.de/en/photo-electronics/mecablitz-models.htmlClick on SCA flash units. I use a model called 70 MZ-5. The new model is called 76 MZ-5, and is better and more powerful.