One of the best spy/infiltration game ever made, Chaos Theory is still amazing 15 years later, one great example of a game that's better than the sum of its parts, wonderfully combine each element of previous entries in the series and expend on them significantly to the point that every pieces just fits and make sense when you got the controller in hand, the music, atmosphere the weight of your movements,the sounds, audio cue from the enemies and the open nature of the level design akin to a sandbox for you to explore and experiment with, it's peak Splinter Cell.
It successfully avoided the inherent flaw of die and retry that plagued the prior games in the series by offering you choices via the all the different paths and ways to tackle your objectives, fluid movements, better gameplay. You don't feel "stuck" in a tedious game moment where you get easily frustrated knowing what to do but having to follow precisely the way the devs intended you to play.
The only areas that could have been improved is the story and voice acting aside of the main cast, Fisher is gold and so his dialogues that even make fun of certain situations, but to give bad voice acting to most of the bad guys trivialize them way to much and fails to make you care about the plot besides the relation with fisher and his team or Shetland.