I vote for the Avengers. Admittedly, not the biggest X Men fan, but read the comics, watched the animated TV show, seen all the films - it's not just a silly preference. All of my favourite Marvel characters reside outside of the X Men setup, with the exception of Magneto.
For me, DOFP does not come close to FC, as FC is ALL about Magneto. His back story, the emotional struggle between finding someone with whom he shares his 'issues' with (the mutation) but also his friendship with Xavier that develops and exists beyond the final scenes and into DOFP. For me FC set the bar very high in the mutant world, as it provided the origins to the X Men films we already adored.
Anyway, I digress. Although DOFP was enjoyable, particularly the 'old and new' characters and all that nostalgia, I think at the end of the film it was overdone greatly. I mean, stopping Mystique killing Trask should have stopped the 'Future Sentinels' from being made, but how on earth did it undo X Men 2? Magneto was still on the loose, and yes Mystique had the change of heart but still... I really disliked that element of it all, was a bit cringeworthy to me. 'Happily ever after'
I enjoyed the film for what it was, action packed and an inspired and risky storyline to take on. They pulled off the future/past elements well, it was easy to follow and understand what was happening in the main for anyone not familiar, and lets face it everyone at least likes Wolverine, so having him there throughout was always going to be a winner.
For me though, it's Avengers all day. Nothing tops the feeling at the end of the film when the Avengers are truly brought together, and yes it may have been comedic at times; there was in-fighting; an egotistical villain that actually thought he could take on a super-team containing a god, a super soldier, a genius and... a Hulk; and yes as a stand-alone film there wasn't a great deal of back story or origin to things, but remember, the MCU wasn't just based on the Avengers film, all that went before it contained the origins, set the scenes, created the platforms and are what contributed to making this movie truly great. You cannot ignore that, as I see some people are doing on here ("what is Loki's motiviation, it's not clear...")
I enjoyed DOFP very much, went to see it twice in a week, but I can literally watch Avengers every day and never lose that feeling of euphoria when the Avengers are truly formed and the future is being created.
Not enough Magneto (never enough Magneto!) vs. all of my favourite Avengers. Sorry Mr Singer, you fall a bit short, but not far short...
Side note, Ruffalo as Banner, and where that can go after two (IMO) awful Hulk films, surely that alone is noteworthy?
Side side note, DOFP post-credits scene: not as good as Avengers, but the most excited i've been since Avengers. Says a lot.