Answer by taking a quote from the following article:

Sunset Boulevard (stylized onscreen as SUNSET BLVD.) is a 1950 American film noir directed and co-written by Billy Wilder, and produced and co-written by Charles Brackett. It was named after the thoroughfare that runs through Los Angeles and Beverly Hills, California. The film stars William Holden as Joe Gillis, an unsuccessful screenwriter, and Gloria Swanson as Norma Desmond, a faded silent-film star who draws him into her fantasy world, where she dreams of making a triumphant return to the screen.

Sunset Boulevard attracted a range of positive reviews from critics. Time described it as a story of "Hollywood at its worst told by Hollywood at its best", while Boxoffice Review wrote "the picture will keep spectators spellbound." James Agee, writing for Sight and Sound, praised the film and said Wilder and Brackett were "beautifully equipped to do the cold, exact, adroit, sardonic job they have done." Good Housekeeping described Swanson as a "great lady [who] spans another decade with her magic," while Look praised her "brilliant and haunting performance."  Some critics accurately foresaw the film's lasting appeal. The Hollywood Reporter wrote that future generations would "set themselves the task of analyzing the durability and greatness" of the film, while Commonweal said that in the future "the Library of Congress will be glad to have in its archives a print of Sunset Boulevard."  The rare negative comments included those from The New Yorker, which described the film as "a pretentious slice of Roquefort", containing only "the germ of a good idea". Thomas M. Pryor wrote for The New York Times that the plot device of using the dead Joe Gillis as narrator was "completely unworthy of Brackett and Wilder, but happily it does not interfere with the success of Sunset Boulevard".  In 1999, Roger Ebert praised the acting of Holden and von Stroheim and has described Swanson's as "one of the all time greatest performances." He says Sunset Boulevard "remains the best drama ever made about the movies because it sees through the illusions." Pauline Kael described the film as "almost too clever, but at its best in its cleverness", and also wrote that it was common to "hear Billy Wilder called the world's greatest director." When Wilder died in 2002, obituaries singled out Sunset Boulevard for comment, describing it as one of his most significant works, along with Double Indemnity (1944) and Some Like It Hot (1959).  The modern review aggregator Rotten Tomatoes reported that 98% of critics gave the film a positive review, based on 60 reviews with an average rating of 9.3/10, the site's consensus says, "Arguably the greatest movie about Hollywood, Billy Wilder's masterpiece Sunset Boulevard is a tremendously entertaining combination of noir, black comedy, and character study."

Who were some of the critics?
Time described it as a story of "Hollywood at its worst told by Hollywood at its best", while Boxoffice Review wrote "the picture will keep spectators spellbound."