I've seen clips from 'Twice Upon A Time', and the paper cut out style was not only unique, but way ahead of it's time. (In fact, it's more elaborate that 'South Park' in some ways.) I've learned how difficult it was to make the film since there was no Flash, and computer animation was still in it's infancy. It also had Lorenzo Music as Ralph The All Purpose Animal.
Another film that's probably forgotten is 'Rock & Rule'. Which was Nelvana's first animated feature. A 'reworking' of their earlier TV special 'The Devil & Daniel Mouse', and much darker, too. It had great animation, great music, and a good story to boot. While Nelvana has become A big player in animation since then, 'Rock & Rule' tends to get overlooked at times. Had it been more successful, it probably would've gotten it's own 'Guitar Hero' inspired game.
Two more forgotten animated films I'd like to throw into the ring are 'A Mouse & His Child', and 'Peter No Tail'.