Just looked at the poll at the top. What was the Twin Towers a sequel to? A World Trade Center movie?
Mannequin Two: On The Move
Duh.
The Godfather II by a landslide. No film ever had to live up to greater expectations, to a greater predecessor. And the fact that The Godfather II manages that and more, indisputably establishes it as the best sequel in cinema history.
Surpasses the original in every single way, and is immensely entertaining:
But of all-time... probably T2.