I can agree that its a tough choice between which of the two movies are worst
I used to love DBZ back in teens - heck, I'm pretty sure that show was why I let my hair grow out. But I grew out of it when I discovered better anime (didn't take much) that didn't waste 70% of its screen-time with useless sidekicks grunting and moaning in awe of how slow the main plot was
the airbender series on the other hand... Oh I loves me some airbender. Good writing, good animation, good character development. It was all good
So, I would argue that the shamylamy's movie is the worst. Why?
The DBZ movie was so bad - and the observation that it looks like a low-budget disney made-for-dvd afternoon special is quite apt...
but the DBZ has one 'saving grace' - its forgettable: Its so bad and goes so much off course from the original material that it would be so easy just to go "screw this, any future DBZ movies ignore what happened in this one" - heck, all the animated DBZ movies prove that DBZ can be done movie length, so just adapt one of those for a movie?
The airbender movie doesn't have that luxury. It was sold as a proper movie adaptation of the series, a lot of money was spent to make and market it... and then it sucked.
Now, while I don't think that hollywood execs would be stupid enough not to recognize that Shamy completely screwed the pooch on this, then I do fear that it completely ruined the possibility for future live-action airbender movies.
in that sense I argue that the airbender movie did the most damage. It ruined a franchize. The DBZ movie was the result of whoever usually wrote the script for hanna-montana being made into writing an anime-adaptation.