Well, in consideration of the theme of the convention. These would be my pics:
Macross Plus - even though Yoko Kanno's http://en.wikipedia.org/wiki/Yoko_Kanno
carrier is absolutely massive. Singing for many albums for amazing anime's. Macross Plus to me sort of iconifies the concept of music-to-anime. Of course it also helps when you have the Israel Philharmonic Orchestra behind you as your making a film too! That and being able to produce two CD's worth of high quality music out of anime series doesn't hurt either. If you get this film make sure it's the Japanesse release as the American one was edited to make little sense.
./hack//Sign - I guess it started off as a video game. Never played the video game, I guess i really got into this anime because of my affiliation with Secondlife and how there is a lot of psychological aspects of this anime which plays out on SecondLife. A bit long for a series, but still . tons of amazing musical scores.
FLCL - I don't feel entirely comfortable suggesting this anime as there would be a lot of new people to anime and this film is possibly the worst thing you could show a person new to anime. However, the garage band style of grunge rock fused with J-pop works well.
Vexille - Some people really do not like hybrid animation because it's -too- smooth. And because of the detail actions such as violence seem stiff and low-key (in comparison to FLCL where it's the absolute extreme with squash and stretch animation) . However, Vexille instead of relying on musicians in just japan they let the handling of the OST be given to some really great artists around the world (my favorate scene was the introduction of the Jags with Lisa Gerard of Dead can Dance signing) A similar film is the remake of "Appleseed" but when it comes to music Vexille is what I would choose.
Well, there you have it, good luck on the video room.