Well, "greatness" in art, of course, is subjective and in the eye of the beholder, but i would suggest that an Oscar is still a fair indicator that a film has both some artistic merit and achieved some degree of popularity and appeal, at least with a mainstream audience. By no means does it signify "best" except by a common poll, but I think it's a safe bet to assume that an Oscar nominated film is unlikely to be terrible and is likely to be better-than-average (individual biases aside of course).
I beg to differ. Here's the list of the Oscar-nominated films. You decide whether they exhibit an amount of artistic merit larger than the films that were not even close to being nominated.
2000 - "GLADIATOR," "Chocolat," "Crouching Tiger, Hidden Dragon," "Erin Brockovich," "Traffic"
2001 "A BEAUTIFUL MIND," "Gosford Park," "In the Bedroom," "The Lord of the Rings: The Fellowship of the Ring," "Moulin Rouge"
2002 "CHICAGO," "Gangs of New York," "The Hours," "The Lord of the Rings: The Two Towers," "The Pianist"
2003 "THE LORD OF THE RINGS: THE RETURN OF THE KING," "Lost In Translation," "Master and Commander: The Far Side of the World," "Mystic River," "Seabiscuit"
2004 "MILLION DOLLAR BABY," "The Aviator," "Finding Neverland," "Ray," "Sideways"
2005 "CRASH," "Brokeback Mountain," "Capote," "Good Night, and Good Luck," "Munich" [the best batch in the last 5 years, I think].
I dunno...