Elesh Norn, Mother of Machines This five-drop 4/7 legendary dork has vigilance, which is better in multiplayer since you can swing at the open player without opening yourself up to multiple counter attacks from everyone
Its sacrifice ability costs two mana, which significantly limits how much you can use it if you also want to cast spells in the same turn
It can essentially cause one of the players to lose as it exiles all cards in their library, but Jaces other abilities are really useful too
So right off the bat, that means that mythics from Worldwake and New Phyrexia are taken care of, and almost certainly means that Jace, the Mind Sculptor is the only Jace well see in this set
The low mana curves present in Modern, Legacy, and Vintage make countering a one-drop for free an absurdly powerful proposition
Flame Jab: solid in Limited