I've been gaming since the Amstrad CPC. Some of the most terrible endings were back then.
Back then you mostly got a screen saying "Congratulations." Or a screen saying that you should try a harder difficulty.
Most recently the ending I was disappointed in ended on a cliffhanger. Where you managed to take down one of the higher-ups in an evil army.
However the person in charge of that same army has gotten away meaning you won't face them until the sequel.
No announced date for release as yet.