id have to say dialga as it beats every mon up there with either its choice scarf or its bulk up set!
zekrom is pretty nifty too, but it loses to dialga regardless of set, groudon if its not mixed (and most groudon do run spdef evs so!), and it gets beat by palkia 1v1 as well.
manaphy actually also does surprisingly well assuming rain up as it can beat everything but the first 5.
mewtwo beats most mons but the scarfers
i want to say giratina-o loses as it can't beat anything 1v1 really besides groudon