Whatever you like fancy or have! I typically use fresh or canned salmon, imitation crab, shrimp, scallops, or canned tuna. See Homemade Sushi Cake for a nice recipe idea for canned tuna specially prepared for sushi. You can enjoy it with raw fish as well, but please be sure to use sushi-grade fish. Sometimes I use raw salmon that I marinate in fresh ginger, soy sauce, and fresh lemon juice, a salmon carpaccio.