I just replayed the entire series over the last couple of months. The first one is the highlight of the series and laid the groundwork not only for the sequels, but pretty much founded the genre of immersive sims all by itself. This type of game is the rare lightning in a bottle that many have failed to capture since and it really hasn't aged much since it came out over 20 years ago. Sure, the voice lines are cheesy, the AI is outdated and there are some pretty wacky characters in this otherwise serious game, but it all fits together extremely well and has a certain charm to it.
I don't think there are any "bad" Deus Ex games, but the sequel Invisible War is definitely the weakest installment. It leans too much into the whole B-Movie theme and, with the exception of the last chapter, suffers from lackluster writing and forgettable characters. The gameplay itself is still fun but overall severely limited due to the hardware constraints of the Original Xbox.
Human Revolution did well to separate itself from the first title while staying true to the core gameplay and I do love the aesthetics that they went for. The story is very solid and I'd say there are more ways to approach a mission than the first game had. Adam Jensen is also a well-written character and a worthy replacement for JC Denton. The only thing I didn't really like was the new melee system.
Finally, Mankind Divided turned out to be the most "Deus Ex" we've gotten since the first game came out. It's a brilliant game through and through and I can't really think of even minor criticism. It's basically what the first game was, just all grown up. Even the DLCs are among the best missions in the whole series.
It's so sad to see the great path this series was on before it was ultimately cancelled again. I felt like they had finally perfected the formula. And now we're most likely stuck with the open ending of Mankind Divided for the foreseeable future.
Check out Core Decay. It's still in development but from what I've seen it follows the Deus Ex formula down to a T. Probably the game I'm most excited about at the moment.