Eh, sales don't determine the way a game in the series is "the best". The reason why New Leaf is so great is because of the vibrant villagers, the way you can be the mayor of the town, and just do stuff. Unlike past games, you aren't working for Tom Nook to pay off your debts, but unlike the game after, the villagers have more personality, the golden axes don't break, you get THROWING BEANS!
But that's just my opinion on that, after all, a game being "the best" is a subjective statement, and every person has their opinions.