I'm still catching up and there's so many great ones, but I loved Nutshell's croaky robot voice in "Because That's Stupid", when reading about the artificial child replacing real children.
Overall though, my favorite is Portaxx's dragon/dinosaur voice.
ETA: Honestly, though it's not necessarily a special voice or anything, I tend to remember the things Isfahan said the most. It was one of the first ones I listened to, but I remember him reading about True Forced Loneliness so well.