Final Fantasy VIII was my first one. It's in the top of the list, but gets beaten by FFX.
Yes, fuck the characters, but it was my first PS2 game. Awesome graphics! They spoke! I was amazed, and also still young enough not to find Tidus and Wakka that annoying.
Remember when FFX-2 was getting released. I didn't know anything about the game except for the title, but I went and bought on the release day. Played for a couple of hours and hated it. Haven't liked any FF game since FFX. Almost considering to get FFXIII-2 though. Only like £11... and I love mah trophies.
Have probably sunk the most hours into FFVIII though. Started the game, got to the prison part, took a break from the game, started over again when I got back to it a few months later. Over and over! Think I only got to disc 3+ once.