IAmSinistar: Aye, the Rebirth mod is a must-have. System Shock 2 is one my most favourite games ever, and you are in for a fantastic experience with it. I love Bioshock, but SS2 is the fountainhead that Bioshock sprang from, and completely brilliant in its own way while also spawning many of the features that its descendant would go on to inherit. The story and antagonists are top-notch as well.
I have to agree with you on the antagonists, they are well-thought out and sinister in their own right, and Sentient Hyper-Optimised Data Access Network is glorious in her own mega-lo-maniac delusion.
I also love how they decided to make the protagonist nameless and void of identity, which allows the player to immerse themselves into it. The story takes off pretty much where the System Shock 1 took off, without spoiling too much and keeping it as a separate story.
System Shock 1 is also a great choice, even though the dated controls can require quite a bit of manual dexterity.