Since Born This Way Ball Tour took attendance in some countries that GaGa has never been before (or any other huge artist) I think ARTPOP tour will be ultimately biggest tour ever because she is now recognized as a biggest touring force all around the globe!
Could ARTPOP be a biggest tour ever?
I think it can because Lady GaGa is one of the few artists that will sell out every stadium/club/arena that she decides to.
Since BTW Ball got cancelled she will have to do a bigger tour to get her money back for her label, employees and her next projects so ARTPOP could easily be the biggest one ever!
Please, elaborate your opinion on the answer "Depends"