Off the top of my head I can think of two series that would have been better if they'd had a proper ending.
1. Neon Genesis Evangelion. An impressive and influential series totally ruined by the mess they made of the final two episodes. Did they run out of money or time, or both, to create properly animated episodes and had to make do with what appeared to be a series of vaguely animated storyboards. Or had they just piled so many plots twists, turns and sub-plots that they'd "painted themselves into a corner" and didn't have the faintest idea how to end the story.
2. Last Exile. Another series with initially a good story idea, but no real idea of how to end the story, resulting in the final episodes being a muddled and confusing mess.
In my view both series suffered from the same fault. A good story needs a beginning, a middle and an end. Both series had a good beginning and a good middle but the creators didn't give enough thought to the ending.