Obviously you didn't see "Junior" because that was by far the worst flick he ever made.
But yes, End of Days and The 6th Day were his worst action flicks...they made me sick.
But I have to strongly disagree with Terminator 3. Yes it doesn't have as cool of a story as the previous 2...but if you just want 2 hours of pure Schwarzenagger action, then that's the best one to see (Not to mention it leaves off with a hint that they're going to spinoff into a 'post apocolyptic' series...of the actual war, which I will DEFINITELY watch)
Though I wonder, did you see Running Man? (Most hate this flick, I love it because of the awesome Arnold Puns) Or...Last Action Hero? Normally when people list his worst flicks they list that one...but I can't remember how good it was myself. (Oh, and Kindergarten Cop?)