By all of them, do you also mean the very first hitman game? That one has aged pretty poorly. The rest hold up pretty well, though.
Hitman 2 has aged quite a bit, but it has some great levels. It also has quite a few bullshit levels where you only have one real option to take out your target. And two of the four Japanese levels are kinda shit, but if you're not worried about getting perfect ratings, you can just kill a couple of guards and make a beeline for the end of the level.
Hitman: Contracts has aged a lot better than 2, and was my personal favourite in the series until 2016 came along. It contains a bunch of new missions plus pretty much all the decent levels from the very first game remade, hence why it might be worth skipping the original. Some people will tell you that the controls are quite clunky by modern standards, and they are, but it feels less clunky than blood money, in my opinion.
Blood money is also a great game and was undoubtedly the major source of inspiration for 2016, but I personally didn't enjoy it as much as contracts. You do have a lot more ways to off your targets than you did in previous games though, so you may like this one more than contracts. It seems that most people do.
Lastly, if you have absolution, maybe play it at some point. It's a good game, but with the exception of a few levels, it's not a good Hitman game. And it's not really necessary to play it before 2016.