Captain America: Civil War, they botched the ideology conflicts in favour of a cookie cutter villain to distract from the actual conflict the movie should have been all about.
It isnt a bad movie, but it is disappointing.
The second I would add is the Warcraft movie, man does it not do the universe justice (down the the incredibly boring main character from Vikings that killed the enjoyment, to the non CGI humans, to the weird plot passing). I cant believe I want more CGI Orcs instead of Humans, they are what made the movie at least enjoyable at some points. How was it so hard to recreate what made the WoW trailers so amazing, the formula was right there.
(Of course Batman VS Superman was a dissapointment but everyone is talking about it so I added the above instead).