FFX was the first in the series I played, and I actually really liked the Sphere Grid. I liked that it was a little bit flexible, and also that you can plot out people's paths in a visual way.

I actually didn't have too much trouble with the difficulty in FFIV DS, but then I have nothing to compare it to (it's the only version I've ever played). Now, talk to me about FFVI GameBoy and the boss battle on the floating continent that has no save point anywhere nearby. :P