It's definitely the weakest in the Die Hard series, but I didn't think it was terrible. When I saw the trailer, I was expecting this to just be a fun action film. From what I got when I saw the movie, I had a decent time.
If I were to rank the Die Hard films, it would go like this:
LIVE FREE OR DIE HARD
DIE HARD 2
DIE HARD WITH A VENGEANCE
A GOOD DAY TO DIE HARD
If they're really gonna make a sixth one, bring back Zeus from Die Hard 3, Holly and Sgt. Powell from 1 & 2, Lucy, and Matt from Die Hard 4. For a villain, get Christoph Waltz, he's a terrific actor. And get Jonathan Hensleigh (director of the Thomas Jane PUNISHER movie and co-writer of Die Hard 3, The Saint, and Young Indiana Jones) to write and direct DH6.