Chronicles of Riddick had pretty excellent voice acting, thanks in no small part to Vin himself. Also, Team Fortress 2--the lines do a beautiful job of perfectly characterizing the person who speaks them.
The conversations between NPCs in Half-Life 2 seemed very natural and were frequently hilarious, especially in Episode 2, when Adam Baldwin played both Sheckley and AR3 Guy. My god, those dialogues were amazing. I really wish Gordon said something every now and again though. It doesn't have to be much, just a confirmation or expression of emotion every now and then. They talk about how making the protagonist talk would break immersion, but the one-sided conversations the NPCs have with you do a far better job of that.
The worst? Hmm, let's see. I suppose it would be unfair to say Mount and Blade, where literally the only lines of dialogue are "Fine day, milohd!" "Less talking, moah raiding!" and "I VILL DRINK FROM YAH SKHULL!" because the developers didn't have the resources to voice-act everything. And the game doesn't really suffer for it.
In that case, I'll go with Just Cause 2. Granted, I believe some of it is intentional, as the game is something of a homage to 80s action B-movies, but it goes so overboard at times that it goes beyond charming and into obnoxious territory. Every time Tom Sheldon opened his mouth, I just wanted to punch myself in the head. "THANKS FER FLYIN AIR SHEEELDON! Hee hee HEH!"