Since Asimov? Since Asimov's generation? Since his birth? passing? or as in...best after Asimov?
Well...there are plenty of good SF (not sci-fi --urgggh) writers in his generation:
Arthur C. Clarke, Alfred I'm a aware of his--Ray Bradbury--work Bester (simpson's reference), Robert Heinlein, and Poul Anderson
Post Asimov Generation: Harlan Ellison, Philip K. ****, Frank Herbert, Larry Niven, Roger Zelazny, Philip Jose Farmer, Michael Crichton, Michael Moor****, and Card.
Within the Post Asimov generation, I'd rate Card well behind Ellison, P.K. ****, and Herbert and in the same pack as the rest on that list.