Honestly 5 had a lot going for it. It removed radio tower puzzles. It way cleaned up on absurd collectaton mechanics of 4, which had gone way too overboard. The survialist bunkers were a neat mechanic to replace a lot of collectaton stuff. I actually enjoyed the side games like the fishing. The gunplay and the customization was iterated on and improved. The editor where you could make your own missions and post them online was really cool (I made a lot of super complicated stealth missions).
The vibe of the game was pretty good, and the villains were engaging enough. It's really just the main plot that falls to pieces and only at the very end does it become impossible to ignore how dumb it is.
I think that in mechanical design, 5 is a straight improvement on 3 and 4.