I got both Deus Ex and Deus Ex Human Revolution recently on discount, and both games are amazing. My only complaint with Human Revolution is that it didn't feel as "deep" as the original story-wise, but it still felt leagues beyond other games.
I am going to play Mankind Divided, but everything I have heard is that Mankind Divided was disappointing so I am bracing myself.
I played the original Deus Ex back when it was new, also DX:HR and DX:MD. Of the two new ones, I find myself going back to replay DX:MD more. To me the biggest problem with MD was it's third act sort of fizzles out, but I enjoy the rest of it enough and the side quests enough to come back to it. Also, the really refined the gameplay to make it more fun.
I am going to play Mankind Divided, but everything I have heard is that Mankind Divided was disappointing so I am bracing myself.