FF VIII and FF X were the biggest pain for me to beat. I just found those two to be so uninteresting and dry that I couldn't stand playing through them. I found myself forcing to push through playing them just so I could get them over with.

Unless you mean which one was a pain in the ass to beat because it was too difficult? If you're going by difficulty, then none of them were incredibly hard to beat. Final Fantasy games in general are pretty easy.