John Ratzenberger, who played the postman who knew it all about nothing on Cheers, has the highest average score of any American actor, according to the article at Slate.com. The actual highest score is some French guy. Of course, Ratzenberger has voiced a character in every Pixar movie made so far, and those movies score really really well on RT's "Tomato-Meter" or movie review scoring aggregator.
The best actress overall is the Armenian-Canadian Arsinée Khanjian, who was born in Lebanon. The best American actress is Amy Madigan, helped by her work in RT favorites Gone Baby Gone and Field of Dreams.
One interesting fact is that, according to the compiled scores, the "most improved" actress is Dakota Fanning, who is all of seventeen years old. Apparently she's a much better actress than she was at 5 -- or at least she's in better movies. Dakota has had an easier road than her younger sister Elle, whose drop between the first movie she worked in (My Neighbor Totoro, 89) and the second (Daddy Day Care, 28) must have been rough. Especially since, according to the score generator Slate includes in the story, My Neighbor Totoro was released in 1988, ten years before the younger Ms. Fanning was born. The discrepancy is solved when we realize that she actually did the voice-over in the 2005 English re-dub of the Japanese animated film, but the graph posts the movie on its original release date.
Cliff Clavin would have known that. But he would have gotten it wrong.