The last story appears to be 1979's Berserker Man, since it tells of the development of a final weapon for destroying the berserkers. Saberhagen continued to write berserker novels afterwards, but (for some of them, at least) there are indications that they do not take place later than Berserker Man. In particular, Berserker Star mentions explicitly that some (but not all) of the events from Berserker Man have already taken place.
I'm not really sure that Berserker Man is that fitting an ending for the berserker saga; I don't think I've ever found anyone who thought it was one of Saberhagen's best books. However, it is the ending he wrote.