AC AF of course:
D'sT is just unpolished (I haven't heard of any bugs/glitches so at least that's something) someone also seems to be having fun with it so it's just a mediocre game.
Mario Tennis: Ultra Smash on the other hand is polished but lacks of content, showing to be a rushed game at it's finest.
AC AF is just bad. the game isn't fun by any mean, you must own AC amiibos to play and even kids seem to not get any particular appeal from it...

Has to be Amiibo Festival. I'm not sure if there's even any enjoyment to be made from it.

As one of the very few, who have actually played the game: The game is a board game. So very much relies on luck. Especially in the beginning. If you play the game multiple times you can expand the board, which creates more tracks and junctions and adds some strategic aspects to the game. The turnip selling, which acts like stock trading also adds a little bit strategy to the game.

It might be a little bit boring, but I still enjoy the game if I am in the right mood. It's kind of relaxing.

Also the game has one mode, which is really great. It's Desert Island Escape. You scan in 3 Amiibo Cards and than you explore a desertetd Island with the scaned characters. Trying to find material to build a ship and escape from the Island. A really nice little mode, which is even more fun if you own multiple Amiibo Cards. (3 are already included in the game.)

If miiverse activity is a good measure of popularity, the games would rank like this:

1. Devil's Third (more than one post an hour - almost two)
2. Mario Tennis Ultra Smash (around one post an hour) 
3. Animal Crossing Amiibo Festival (one post every second hour) 

I voted for Amiibo Festival. And no offense, but if I have a "prejudice" against the game, what about your own agenda? You've been aggressively promoting and defending the game just as much if not more than I've been criticising it. Can't we just say that you like the game, I don't, and leave it at that? 

And I reject the idea that you have to have played Amiibo Festival to judge it. Half the problem is how insulting and transparent a cash grab it is, and you don't have to play it to see that.

Amiibo Festival. The rest are crap too, but it seems that they took more work at least.

Amiibo Festival takes that spot easily, followed by Mario Tennis in second and Xenoblade Chronicles X in third place.

Easily Amiibo Festival. I don't even understand why it exists..

Amiibo Festival hands down.

Atleast the other two TRY to be a game, with game play, and stuff. Unlike Amiibo Festival that is just a stupid piece of software made purely to justify the existence of a new amiibo line.

Probably Devil's Third.