Sad-ass sequels: "Caddyshack II", "Blues Brothers 2000", "Highlander 4: Endgame" (truly an awful movie), "Jaws 4: The Revenge" (don't ask why the shark killed in Jaws is still around for this one), "Weekend at Bernie's II" (if you like watching corpses being dragged around, Bernie's I was okay), "Beyond the Poseidon Adventure", and last but not least, "Speed 2: Cruise Control"
Best non-mafia sequels: "Aliens", "Star Wars - The Empire Strikes Back", "The Bride of Frankenstein", "Terminator 2"