The original Half Life, (which, BTW, is free on Steam at the moment for those young whippersnappers that have never played it), was a masterpiece of aural immersion - particularly for the time it came out.

I downloaded it this weekend (as its free and god knows where my original discs are) and immediately got all goosebumpy at the ambient sounds - even in the training course.

All the environmental effects, echo, distortion, occlusion, the radio chatter, and even the little skittering insects and things you can hear scurrying about were unheard of (excuse the pun), or at least not done so well, at the time, and they still hold up today, 25 years on.