It doesn't have to be a particularly bad fight, merely the least appealing to your tastes. Mine is Deidara and Sasori Vs Kankuro and Sai. I expected so much from this battle, but it was over in a flash, with Sasori pretty much choosing to die. that was irritating.
But that is only second worst. the very worst battle i have ever seen in naruto is sakura Vs Ino.