Strictly personally as a Hitman fan, I would of course recommend playing all of them. They all have their quality and charm, though they of course come with both pros and cons. But it largely depends on how ok are you with playing old games like these. I´ll try to break them down a little for you:

Codename 47 - I don´t think it´s more of an action game. It´s just more on the "B movie, almost comic book" side of things in terms of story and characters. There is only one shootout-only level, plus a few levels (specifically the Rotterdam section) where you are required to shoot some "non-targets" in the process. Otherwise it´s a combination of sneaking and disguises, and certain potential action sequences (Colombia) can be easily avoided. It´s also lighter on the options side, with each mission/target usually having one or two "correct" ways of killing them (though there´s of course an abundance of ways for creative players). If anything, I would call the game somewhat challenging. The gameplay and controls are a bit clunky and not as refined as in the later games. The map doesn´t show NPC positions, so its usefulness is limited. And there is no save option, only a checkpoint system of sorts, so the game can get a bit frustrating and punishing before you figure out the right strategy. But it´s also one of those games that feel very rewarding to finish (and gets much easier the more you play it).

Silent Assassin - My personal introduction to the series. Improves on C47 in every way. Gameplay and controls are much better. It caters a lot to slower, methodical gameplay and stealth. It has a proper save option and introduces the mission rating system to the series. You also have an indicator of how suspicious enemies are of you (I recently found out that a lot of players dislike the game for the AI being overtly suspicious and unpredictable in terms of when/if they will attack you - it´s kinda true, but I personally never had an issue with it). Map now shows NPCs. Most importantly, many of the levels are more sandboxy and allow for bigger freedom of approach, though there are still the more linear "build-up" levels as well, some of which are poorly designed and universally hated (Hidden Valley as an example). But otherwise I think it holds up great.

Contracts - Basically takes what Silent Assassin did and improves on it. Same mechanics, but gameplay is even smoother (e.g. the sneaking is faster and the shooting is better, if you decide to resort to it). The enemy AI is less "unpredictable" than in SA. Also, whereas the previous two games had a lot of "build-up" levels before the actual hits, in this one, every mission apart from one has targets. One aspect which can be both good and bad is the game´s atmoshpere. All levels side one take place at night with either rain or snow. This on the one hand gives it a unique dark and gritty atmosphere for which many love it, but also makes it visually rather monotonous. The levels offer even more freedom of approach, but it´s worth noting that half of them are remakes of those from C47 (the vast majority of missions here are flashbacks).

Blood Money - My personal favourite of the bunch (along with SA). Every mission is now its own "contract" with targets, some of which have some nice gimmicks. You get a lot more tools to play with and even some distractions to use. Implemented the option of environmental/accident kills as well as new mechanics (elevator kill with fibre wire, human shields...). Has probably the best sanbox elements of the four and adds in some extra stuff such as (limited) weapons customization. The gameplay is smooth, though some of the controls aspects could be better. For whatever reason, shooting (at least in 3rd person) is worse than the previous game though... And I´d also say it´s more lenient in terms of rating/what it lets you get away with. Also, minor warning: the first mission is a very linear tutorial that introduces you to the features, so don´t get dissuaded by it, cause the other levels are nothing like it.

Basically, other people´s opinions about which game is better or worse boil down to their personal preferences (and often also which game they played as their first one). I know people from the Hitman community who will equally defend one of these as the best of the classics, while finding flaws in the other ones. I would say the basic gameplay in all four is pretty much the same - explore the area, obtain disguise to get somewhere if needed, sneak around if needed, neutralize enemy if needed, kill target(s) (if any) in a way that does not attract attention, get out - but with improvements along in each installment.

I suggest you try all of them. If you´re not sure of what to expect, watch a gameplay video of a mission or two on YT to get an idea of whether you´d enjoy it or not. And if you try one and it doesn´t quite suit you, don´t let it dissuade you from trying out the other(s) ;-)

Hope this helps. If you have any more questions, feel free to ask!