It's a toss up between Suteki Da Ne and Real Emotion. I like all 3 of the songs, though.

Suteki Da Ne played during one of the most memorable scenes in FFX and it helped convey the emotion perfectly.

I remember the first time I plugged in X-2 and just being amazed by the graphics at the time. My dad walked in and said "what are you watching?". Couldn't believe that those weren't real people on the screen. Real Emotion is a nice upbeat song and sets the tone for that first area of the game.

Hard choice.