I think The Day The Clown Cried is probably the winner, though clips have now gone online and I think the entire movie will eventually be viewable? It was, I believe, finished and edited. Not sure if entire post-production was done, but the movie is supposed to be viewable at some point.
Last year, we had Batgirl, which I believe was actually not completed. Rumors abound about how close to completion it was, but I think it was about 75% filmed and no final edit obviously exists. Effects and so forth were not fully added, though rumors persist the movie was actually terrible.
I’ll add a less discussed one. Eddie Murphy filmed an entire pilot for the new Beverly Hills Cop show where he reprised Axel Foley and did a lot more than just cameo in the episode. Problem? They talked to him about appearing a few times a season and he flat told them no. He would only do the pilot, which he had already filmed, and once they realized he wasn’t going to come back…they canceled and canned the episode. I presume no one ever saw it? No release or clips sent out, I think.
What are the big other “in the can” productions that remain by the larger world?