Voted for monsters because it's my main deck at the moment. I have also used Skellige a lot. I keep changing my decks when I get new cards or get bored with current strategy. Or I just may try something crazy and continue with that. So my "main faction" changes all the time.
There's no "All of the Above" option. I have 1 deck for each leader and I try to play each of them at least once per day.
Now, if it was favorite decks, that would be a toss up between Brouver's Dwarves, Ge'els' Golems and Foltest's Blue Stripes. While I enjoy all three of my Skellige decks, I just have not yet found that theme that resonates for me yet (though I am getting partial to my Skjall deck, it just needs a more unified theme).