I just finished listening to Shantaram, by Gregory David Roberts, narrated by Humphery Bower, and I have to say I spent every one of those 43 hours completely enthralled with his voice.
First of all, he's Australian (appropriate for the first-person protagonist of the book) so everything is delightfully accented, even just the "base" narration. But it's the character voices that always really sell me on a good narrator. Most of the book takes place in Bombay, so there are umpteen different Indian characters, all with distinguishable, recognizable voices. Aside from that, though, there are numerous characters from other countries, with accents as diverse as French, German, Spanish, British (London and Liverpool versions), Canadian, American, Iranian, Pakistani, and Afghan. And as if juggling all that weren't enough, he also handles cases like British-educated Afghan, New York Pakistani, and an Australian faking an American accent while speaking Hindi. The guy is a genius.
And even beyond the accents, the overall narration is just exquisitely done. Every little shading and coloring of emotion and meaning is conveyed perfectly in the intonation. Not too much -- I've heard narrators that overdo it -- but just right. This goes both for matching explicit descriptions in the text and for applying his own interpretations to the rest of it. There have been a number of times when I could tell that I would have read something in a very different way in my head, but that Bower's version worked much better.
So if you're a connoisseur of good narrators, definitely listen to this. (Or I expect any of the other books he's read would be excellent as well.)