Primedice is the Highest paying program now considering Updowns problems and that Fxopen is closed to new participants.
Less the fixed price ones in some cases.
That said I am not really concerned as an Fxopen user about them not paying since they are an established exchange around since 2003 and I don't see a reason someone not in their company would bother making a sig campaign for their service.
http://en.wikipedia.org/wiki/FXOpen