Peter Kenny reading The Witcher series did an amazing job doing unique voices for every character in an enormous world. I could listen to that series over and over.

Also Simon Vance reading Clive Barker's books is an amazing voice narrator. I can tell which character is speaking without it even being said because Simon, like Peter, does an amazingly unique voice for each character.

Lastly, Ralph Lister reading the Malazan series does a pretty good job, but I haven't actually dove into that series yet, but really like what I hear from samples of his narration.