Deus Ex is one of the greatest game of history. Be it the very first one or the third one (Human Revolution), the mix of RPG, FPS, Stealth in a cyberpunk world is just so wonderfully crafted, you feel totally free of the way you play the game. I have many fond memories about the first one, and they are all memories I crafted for myself by the way I approached some events/challenges, and the way it turned out to be a good idea and a lot of fun to accomplish.
Human Revolution was almost as great, except for the boss fights which were frustrating, but were improved in the Director’s Cut edition of the game. The first Deuas Ex’s excellent Game Designer, Warren Spector (Thief, System Shock, Ultima Underworld) has been invited with two of the Deus Ex dev team (the lead developer and the dialog writer) to play and comment on the game during one very interesting hour. Here is the video:
For more information on the game creation, I redirect you to an interview given by the same protagonists to Gamasutra:
And if you want to know the feeling you can/would get by playing it 15 years later, here is a long but interesting series of articles on Rock, Paper, Shotgun: