I've only played Human Revolution so far, but I loved it. I'm heavy into the cyberpunk themes and foreshadowing what technology could look like in the future, especially with the ever advancing research in biotechnology.

I'm sure I'll play the rest of them one of these days. You'll occasionally find them on Steam sales for 3 dollars a piece, maybe less in bundle packs.

Here's a treat:

The Deus Ex Facebook page gave away the Fallen Angel digital novella for Christmas. They put up a link. It just pops up as a pdf right in your browser if you'd like to read. It tells the story of Faridah Malik prior to the events of Human Revolution.