I'm not quite why sure there's been such a huge reaction to the film version of X-Men: Days of Future Past (DoFP). As of this writing, the movie has a 91% aggregate score from critics on rottentomatoes.com and an absurdly high 95% from audiences. That's better than Captain America: Winter Soldier (89% and 94% respectively)!
Now don't get me wrong, I enjoyed DoFP and thought it was entertaining enough. However, it is nowhere near as good as Winter Soldier. That movie had everything going for it. A great tone, script, pace, acting, direction, humor, choreography, effects, continuity, fan service, respect for the source material, respect for the audience, respect for the filmmaking art... hell, it even had Robert Redford!
While DoFP does have some good things going for it, it's not the complete package that Winter Soldier was and while I realize it's a subjective thing, I don't feel it deserves to be called the "best" X-Men movie.
Here are some of the reasons why.