I rewatched them all but TDP a few months ago and I do not agree. The first is the best but I think the other three get progressively better.
Mangum Force is a good action movie, but it waters down Harry and makes him normal.
I remember thinking that The Enforcer was the weakest but I like it now. There are some funny parts and the chemistry with his female sidekick is good. I don't know why she's allowed to wear high heals though, and the villains are lame.
Sudden Impact alternates a comic plot where Harry seems to be causing more mayhem than he's stopping, and a more serious story. Both of them question him as a character, and that was where Eastwood wanted to stop, but they made him made another one. I remember Dead Pool being an amusing f-you movie, but it's not a satisfying series entry.
I just watched Cry Macho and it's feelgood fluff, but it's way more likeable then some of the crap he made recently that garnered Best Picture nominations.
Also watched Deadly Friend, which I guess was maimed pretty badly by the studio, but it still manages to effectively make an 80's teen horror equivalent of weirdo 30s horror movies like Bride of Frankenstein and Mad Love.