You beat me, I was going to say the Rambo sequels too. Fist Blood was great, but the sequels after it were so stupid. I did however like the last Rambo. But I heard, it might be just a rumor, but I heard that Stallone is working on a true sequel to first blood, in which John Rambo is called back into active duty to hunt down a young special forces veteran of Afghanistan who has a PTSD episode and goes on a rampage in some small Washington State town (that will of course be filmed in B.C.) Essentially bringing the whole franchise full circle. Which is kind of cool, unfortunately that plot has been done so many times since First Blood. Especially in the 2003 movie The Hunted with Tommy Lee Jones and Benicio Del Toro, which has a similar plot to what I just described.
As to what I think of as the worst sequels ever. Nothing can beat movies that are so bad they completely destroy, or at least derail, huge successful film franchises. Like: Batman and Robin, Alien Resurrection, Star Trek: Nemesis, Die another Day, X-Men 3, and Spiderman 3. Movies so bad there was no repairing the damage those films did to their franchises necessitating full resets/reboots/prequels. It's not all bad though, after all we did get the Christopher Nolan Dark Knight trilogy (awesome), the Daniel Craig Bond movies (so far so good), X-Men First Class (great), Ridley Scotts Prometheus (Ok), The J. J. Abrams Star Trek (meh...), and the Amazing Spiderman (whatever). Which we wouldn't have gotten had those franchises not hit rock bottom before hand.
Another terrible sequel that other people have already mentioned, Indian Jones and the Kingdom of the Crystal Skull. But at least that piece of shit movie gave us a term for the film equivalent of "Jumping the Shark": "Nuking the Fridge"