Mother of Runes
 
I am a fan of all the Masters sets. You'd think that a game with 20 years of history to draw on would be more eager to actually do so; instead, we have occasionally been told that Magic must always look forward and that new sets are about "fixing" things. But it's a great sign that Eternal Masters is here now, because reminders of where we came from can provide inspiration for where we - in a personal sense, as gamers - might go in the future.
 
Mother of Runes might be one of, if not the, most deceptively powerful creatures in history. A white 1/1 looks painfully easy to kill. But with this card, you sometimes only get one chance, and letting the opponent untap with her in play means you may not be killing anything, ever. The only wrinkle I can think of is that she is in fact a 1/1 creature, and can sometimes be slightly hard to fit into speed-based white decks. But you should do so anyway.
 
Constructed: 4/5
Casual: 4/5
Limited: 4/5
Multiplayer: 4/5
EDH/Commander: 4/5