Monsters, however purely by coincidence.
Here is the story:
I am quite new player. I went to rank 25 about 3 days ago, so I finally got leaderboards rating revealed. I checked, that Northern Realms and Monsters are my worst factions regarding rating. I managed to get to rank 24 and there, I was going forth and back, even falling back to rank 25. Today I was thinking, ok, I can not climb higher for the moment, so let me just play six 1:2 lost games, so I will get some reward because of 6 won rounds. And what faction will I choose? I do not want to hurt any of my higher rating factions, because if their rating will fall down, I will have to win even more games, to improve rating of my best factions. So lets play the least favourite faction with the lowest rating and that is ... Monsters. I could not believe it, but I miraculously went 8 consecutive wins and completed rank mosaic 24 and 23 and set my foot into rank 22 with Monsters faction having currently highest leaderboards rating amongst my factions. Simply unbelievable. I still do not know what happened. Was I playing at special time, when only weaker players play? Was there some mysterious gap in rating, so strong player concentrate around ranks 25, 24 and after that it is easier to advance? Was there some change to rating? I noticed, that faction ratings were getting +1000 points update for each faction in Sunday 11/11. Whatever was cause, Monsters went from my the least favourite faction to the most favourite faction.