Sonic SatAM. This is how Sonic was meant to be, not the corny crap they have nowadays. This one actually has a deep storyline with real awesome action, awesome art that ended prematurely in favor of the slapstick crap version of Sonic.
Dungeons and Dragons. Another awesome old cartoon/anime that never had a true conclusion of the kids being able to return to their world. I hate when storylines leave loose ends.