FFX-2 is by FAR the easiest one, based on plot battles only. Umm, Alchemist, anyone? The optional dungeons BARELY make it worth playing through, but more people will be more upset based on the fact that the optional dungeon isn't difficult, but is rigged.
Rigged == random damages over 10k, without anyway to guard, evade, or get enough HP to withstand without compromising your attack, which you need to actually defeat the enemies that DO that much damage.
FFVII is marginally harder, but with KotR, all == dead. For me, I found it hard, but only because I missed the Life Materia, and never bought Pheonix Downs. If Cloud died, he stayed dead.
FFIV (FFII, NA), DoS I and II are beyond easy.
X isn't easy because of Seymour Flux and Yunalesca... alone. Even the cutscene right before them, which you can't skip, is designed to make you tired, and rush through without thinking, which == dead.