Deus Ex - Objectively best game in series, one of the greatest games ever made, hasn't aged all that well.
Deus Ex: Invisible War - A big let down compared to the original. Areas were too small, the non-shooting gameplay was less interesting, the story wasn't great. Still a really good game, but a let down compared to the others.
Deus Ex: Human Revolution - My personal favourite. Sacrifices some of the freedom of the original for improves moment to moment mechanics, especially in terms of stealth action. My main gripe with it is the XP system; the game is most enjoyable if you don't try and min/max and just play the way you want to.
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