Greatest Living Actor? Actress?

While trying to pass time at work yesterday, I came across a blog that said Robert DeNiro is the greatest living actor. (I unfortunately don't remember where I saw it - some down-home blog like this one. Nonetheless, here's a poll from 2004 that also said it was DeNiro, followed by Pacino). I didn't think much about it at the time, but it's an interesting question.

Is DeNiro really the best living actor? Certainly, his work at its peak (Taxi Driver, Raging Bull, Goodfellas, Heat) is among the greatest ever, and he salvaged a lot of movies that had no business being anywhere near as decent as they were (Awakenings). But he's done a lot of crap, especially in the last 10 years (Righteous Kill, Godsend, The Adventures of Rocky & Bullwinkle). I realize some of this is terrible script-writing and/or directing, but it's also terrible choices on his part and not exactly amazing acting. Ditto Pacino. Does that recent history take away from his status? Are there others out there who perhaps have a relatively smaller body of work, but of a higher overall quality, like Daniel Day Lewis or Philip Seymour Hoffman?

So - who is the greatest living actor?

And is Meryl Streep the greatest living actress? (I have a harder time coming up with a good alternative here, though I could just be tired).