Too tough.
Even if I break things into categories, I can't decide. Is the best comedy The Death of Staline, The Names of Love, Pee Mak or One Cut of the Dead ? Is the bloated pop-corn flick of the decade Baahubali, Battleship Island, The Taking of Tiger Mountain or Assassination ? Is the best "instant classic" drama The Son of Saul or In The Fog ? Is the best Zeitgeist movie Margin Call or Joy ? Is the best overlong epic Mysteries of Lisbon (5h30), From What Is Before (5h38) or La Flor (13ish hours) ?
Inspired by wormie that went with an arbitrary but objective criterion (in wormie's case it was "how many times have I watched it"), I think I'll use "How often do I think about the movie." By this metric, the winner is a chinese neo-noir that is not even the best chinese neo-noir of the decade (that would be Black Coal Thin Ice). So the winner is, the borderline experimental and supremely dry People Mountain People Sea. About 30% of all the spoken lines of the film are in the trailer and I am not even joking.