Some movies are great and hook you but then they just fall apart towards the end and end up dragging down the rest of the film.

What films are like this for you?

My list:

Star Wars Return of the Jedi- Ewoks. Worst Climactic Lightsaber fight of the saga.

The Blair Witch Project- Just show us the damn witch!

Interstellar- You lost me at the bookshelf.

The Little Prince- The source material is not enough for a 100 minute film.

Donnie Darko- Was a cool movie about a teen suffering from mental illness ended up being a wacky sci fi pretentious crapfest.

Harry Potter and the Half Blood Prince (The whole film sucks)- Snape brings no emotion to the climax. No cool duels.

I liked the saber duel especially when Luke loses it when Vader threatens to turn Leia, but i agree the Ewoks killed it for me.

I actually thought the last act of the Blair Witch Project was done brilliantly. The point was not to "show the Witch" because the core concept of the film is that there's no guarantee there even is a witch; it's about presenting inconclusive evidence to the viewer so that our imaginations fill in the gaps.

Anyway, some that come to mind for me are Gone Girl, The Descent, No Country For Old Men, Oldboy.

I agree. The new one showed the witch and it just wasnt the same.

Wild things.

And Donnie Darko ending was great, it explained all and provided a solution, while not being the super-duper-everything-is-all-right-Hollywood-ending.

I do accept what you said OP it's so cool :) keep going

Blair Witch is one of the most boring films I ever did see beginning to end.

First thing that pop into mind is obviously Interstellar. The film is set up as one of the greatest of its time and end as shitty as possible.

Die Hard With a Vengeance. A lot of people consider it the best next to the first one - A statement I agree with - The ending is what keeps it from being as good as the first one. Its just... Not a good enough ending. It just ends and whatever. Using a helicopter should be consider cheating. 

Logan - Imma go with Logan simply because, however great the film is - the ending is terribly predictable, cliche and by the numbers and I just personally dont like when you see the ending from the 1st minute of the film. I dont know. I dont think it was bad, it just wasnt... was obvious, not as good as it couldve been. 

Sunshine. I guess its the obvious choice for good movies with bad 3rd act, but when a great sci fi film is turned into a bad cliche slasher film ... well, just lol. 

Disagree entirely with that statement. 

metal gear solid 4, nano machines and wedding

Sunshine is pretty much the ultimate example of what you're talking about. So much wasted potential.