Dirty Harry and Sudden Impact are probably the darkest films of the franchise. And the best ones with Magnum Force.
The Enforcer and The Dead Pool are more formulaic and work more like TV films. Magnum Force has that aspect a little bit too. Obviously, the five films of the franchise contain some sequences of blood and violence.
However, Dirty Harry and Sudden Impact seem a lot more depressing and melancholic than the others.
It may come from the cinematography of Bruce Surtees who worked on Dirty Harry and Sudden Impact but not on the others. It also may come from the acting performance of Clint, who played the bitterness very well in Dirty Harry and Sudden Impact. Do not forget the great directions of Don Siegel and Clint who managed to introduce Harry Callahan, more lonely and sadder than ever. If I am correct, in the USA, Sudden Impact is famous for the Make my day scene but not really praised by the film critics or the cinema lovers. However, in France, Sudden Impact is generally considered as good as the original film, if not better. In my opinion, Dirty Harry remains the best film of the franchise: Clint delivered one of the best acting performance of his whole career, Scorpio is one of the best villain of film history and Don Siegel succeeded to make an other great crime film set in San Francisco, after the unforgettable Bullitt. However, the fact that Clint directed Sudden Impact, helps to make it more than just an other sequel. I think that Clint brought his own artistic sensitivity: something missing in the other sequels. The final sequence of Sudden Impact is particularly powerful.