It's not actually my favorite (that goes to Winter Soldier, hands down), but in my opinion it's their best work. The script's pacing is perfect, all of the characters get enough time for the audience to build a connection with them- no easy task with an ensemble film but they made it look effortless. And while outside of the comics world the characters are a bit obscure, the adaptation managed to win people over. Again, no easy task. Not with so many other movies competing for attention.
It's amazing because in some ways it follows the same formula as the Avengers (and a bunch of other team-up movies) but it's different enough to not bore people.
Not to mention the amazing cast.
And the design team!!! Everyone who worked on the movie was incredibly passionate and excited about the story they were telling. They were clearly having a lot of fun. And it shows. Marvel really needs to diversify- not just with their casting but with the stories they're telling. They can't deliver the exact same movie over and over again. We'll get bored! Guardians proved that changing things up just a little isn't a risk- it's quite the opposite. Which is why I think it's their best work to date.