since the Cosina Bessas have no TTL flash, it does not make a lot of differnce which flash you use. the first place to look is the flash atop your own SLR. IF it offers auto exposure from a sensor on the flash, try that. Personally I like the high end Nikon flashes with variable f/stops, variable manual power, zoom flash coverage, and rotating flash heads. Tajke note that the discontinued and relatively inexpensive Nikon SB-25 or 26 offers all of these, you don't need to buy Nikon's latest and most expensive flash.
Stephen