I've played all of them, but by far, New Leaf is my favorite. It is very different from the other games, and the art is beautiful. In the older games, things were so... pointy. Now things look much smoother. I also didn't like the way the camera moved in ACG. I am happy you don't need a gameboy to go to the island. I love the new species added. They give the game much more life and character. I also like how they changed the player model. Making them taller, and allowing for hats and other articles of clothing. Overall, New Leaf gives something different to the series, and I enjoy it the most.
My favorite has to be New Leaf. I thoroughly enjoyed all the others, though there were some parts I couldn't play, due to lack of a Game Boy Advance. I wasn't able to explore all that Animal Crossing had to offer until New Leaf.
As great as New Leaf is, I have to say the original gamecube animal crossing. There's just something about it that the other games can never top. Nostalgia maybe?
New Leaf is the best Animal Crossing game and there's really no argument there. There are still some things I think that the GCN version did better, and I personally enjoyed the GCN version more... but I was young and it was my first Animal Crossing. New Leaf wins by a longshot.