Codename 47: That game was probably the biggest pain in the ass to complete. The Lee Hong Assassination took me like at least 50 attempts before I cheesed it and used the god mode cheat to beat it. A lot of the difficulty just comes from the clunky, outdated mechanics and bad AI.
Silent Assassin: A lot more fair than the first game, the challenge of walking around in a disguise is actually pretty cool most of the time and generally when you screw up it’s your fault. That said the AI is really inconsistent here and there’s no intuitive way to figure it out - the best example of that is the Japan missions, where the guards can blow your cover from miles away but as soon as you enter the actual castle your disguise suddenly becomes perfect. Also there’s a lot of (more or less) forced sneaking sections which are just… ugh
Contracts: Here the guards are less attentive than the ones from SA, but their AI is just as inconsistent - you never know how close you can walk up to a guard without them seeing you and there’s a LOT of moments where you have to get really close to an NPC. Whether I fuck up or not seemed to be based more on dumb luck than any actual skill. Also it seemed to be the most glitchy out of all the Hitman games (maybe except for the first one).
Blood Money: Not a lot of people remember it, but that game got a lot of flack from the fans when it came out for being “too easy”. I can’t say I agree with that - sure, if I was in charge of level design I’d make some of the kills more challenging but overall I’d say the difficulty in Blood Money is fairly balanced. Hell, I’ve played every level hundreds of times and I still fuck up a lot. IMO, ditching the suspicion mechanic was a good idea - it always felt like annoying pseudo-realism (apparently this bald white skinhead can totally blend in with a bunch of radical islamists as long as he doesn’t walk up too close to them).
Absolution: I really don’t like this game, so this might get a little ranty. Absolution is really hard if you try to play it like an actual Hitman game; the most efficient way to get through most missions is to get a disguise, crouch-walk everywhere and roll from cover to cover (which apparently no one finds the least bit suspicious), occasionally walking past guards using 47’s magical “instinct” powers. It makes even less sense than H:SA (at least there the levels were big enough that you generally didn’t have to get close to the guards at all) and it looks ridiculous. Also, the levels where you actually got to assassinate someone were disappointingly easy - I got Silent Assassin on my first try every single time and I’m not even that good of a player.
I haven’t played the new games yet, but from what I’ve seen so far they do seem to have a really good approach to difficulty, where you can make the game as easy or difficult as you want it to be (e.g. by disabling the hints regarding opportunities). For now, my vote goes to Blood Money.