I'd say Nelvana's first film effort, Rock and Rule (1983). While the story has some flaws, the animation, music and atmosphere are great. I find it really entertaining to watch, and it's got some really good voices in it (Susan Roman as the heroine, Don Francks as the villain, Catherine O'Hara, even Maurice Lamarche in a cameo part).
I understand it's dated and definitely not everyone's cup of tea, but it hardly ever gets mentioned anywhere.