I loved the theme changes on them -- I remember HK Streets had explore, combat and death scene; transitions felt seamless in the game.
And the discoverability of all kinds of stuff in the canals. The cityscape building hopping and secret back entrances to the duplicitous Maggie Chow's apartment. It just had everything. You truly felt like you were discovering every inch of it and like it wasn't just on rails or procedurally generated.
I actually got chills during the surprise post-credits scene in Deus Ex: Human Revolution when the og Deus Ex theme started playing.
Shame what happened to the series after DX:HR (with Mankind Divided feeling rather unfinished and ending on an abrupt cliffhanger with no follow-up on the horizon, despite the game itself being great otherwise). But at least we got some really good crumbs (HR and MD) out of it, and I am very much thankful for that.
I can recommend the HD remix.