Best narrator, period?
Hard to answer. Some narrators are better suited to some tasks than others. Dan LaFontaine was a marvelous movie trailer narrator, Morgan Freeman is a good movie narrator, Stephen Fry is a good documentary narrator (so is Sigorney Weaver, believe it or not; she was wonderful with the TV broadcast of Planet Earth), Norio Wakamoto is a good <url=http://tvtropes.org/pmwiki/pmwiki.php/Main/LemonyNarrator>lemony narrator (as is whoever that guy was who did the PS2 Bard's Tale game, and, oddly enough, Crispin Freeman)...
I think, just out of loyalty, I'm going to have to go with Christopher Sabat, for his spectacular(ly hammy) narration of the Negima!? anime.
"Shock and awe! Has Negi finally met his match?! Look forward to the next episode with NAIL-BITING DESPERATION!"