Like, opening, as in before you press start, or the time between pressing new game and actually playing?

For the former, I gotta go with The US opening for Tales of Symphonia. The sweeping orchestral arrangement along with the very nice animation leads it to being one of my fondest openings ever. I'm also fond of the opening for Fire Emblem 7, the "Aria" playing during the opening was especially impressive to me for a GBA game. Also, Twilight Princesses opening was the original E3 trailer with clips from the finished product, so as someone who spent 5th grade drooling over that trailer, that was especially cool.