Best NPC chatter!
One thing that always enhances my enjoyment of games is the ambient NPC chatter. Really good, spontaneous chatter really immerses you in the experience, making a game feel more organic, less scripted. Even something as simple as a change in inflection from unit acknowledgement goes a long way towards building the experience. Relic and Valve are especially good at this, among many other companies. That said... here's my tops!