Batman Forever
Van Helsing
Mission Impossible 2
Ghostbusters 2 (I saw it before 1, which might have affected my view)
The Three Musketers (2011 version)
Slither (if you count it as being panned)
The Underworld series, particularly 2 and 4 (3 was a bit shit even for me)
Species I and II.
Possibly Van Helsing I wouldn't quite defend to the death, but it was a fun romp that I don't think should have been quite so panned. As for Slither, Species II and Underworld Awakening... I'm a huge fan of both the genre and those movies. I think reviewers have a tendency to get bogged down sometimes, and it gets worse when it's a genre they don't like. Species II has some very nice direction, a plot which is at least engaging enough to get you through the whole movie, it provides good to middling actors with a chance to just have fun, and it shows, and purely as a fan of shlock it delivers titties and gore. While it may not compare to The Godfather or Citizen Kane that doesn't necessarily make it a bad movie.
This I think happens a lot to sequels. Just because it isn't as good as the first one doesn't make it bad. Mission Impossible II is a stylish, slick action movie with a MacGuffin driven plot and plenty of John Woo awesome. It is not as good as Mission Impossible I, but it stands head and shoulders above so many other blockbusters. There are movies out there that will send you to sleep they are so boring. I saw Michael Bay's Transformers on DVD with some friends and ended up taking a bathroom break in the middle of the climatic action sequence because I found it so boring it was a choice between going to the toilet or going to sleep. Don't just compare sequels to the other movies in the series, compare them to other movies in the genre and realise that you might be watching a substandard Harry Potter or Bourne Adjective, but you're watching a movie that is better by far than any other example of the genre. (for the record though, I hate the Bourne movies so much)