Selena Gomez -- My Famous Friends Are Hotter Than Yours

Julianne Hough and "Vampire Diaries" star Nina Dobrev are two of the hottest chicks in Hollywood ... and both were at Selena Gomez's 21st bday party ... but the question is, who's hotter?

Nina Instagrammed the pic Saturday night mid-celebration of she and Hough dressed as gypsies -- per Selena's theme -- and exposing their midriffs ... making everyone with eyes super happy.