Liking Robert Indiana’s art has always seemed like a guilty pleasure. That’s partly because “LOVE,” his iconic work, is hugely popular, and also because he expressed sentiments—LOVE, HUG, EAT—that verge on the sentimental. But, “Robert Indiana: A Sculpture Retrospective,” organized by Joe Lin-Hill, the deputy director of the Albright-Knox Art Gallery, where this exhibition closes on Sunday, makes a compelling case for the stature of this late Pop artist. Walking through the Buffalo museum’s show, it’s clear that Indiana wasn’t a one-shot wonder. He should not just be identified with his ubiquitous “Love” series—all of the paintings, sculptures, drawings, and prints he executed in dynamic capital letters with a slanted O.