Here's my current watchlist, organized in order of severity (In my opinion):
Wurmcoil Engine: 10/10
It's hard to think of a card that stops aggro colder. This, combined with the death trigger makes it incredibly hard to deal with.
Gideon Jura: 9/10
My cube has a high density of creatures, so Gideon is hardly ever +2ing on an empty board. Forcing your opponent to attack and making all the best trades turn after turn is incredibly crippling, let alone just being the best nekkrataal ever and beating for 6.
Grave Titan: 8/10
Nothing answers him but a hard wrath, and he is an incredibly fast clock.
Coalition Relic: 8/10
Maybe it's the density of insane 6 drops, but skipping ahead two turns like this AND fixing mana is insane. Also it synergises well with green cards, since the only thing more impressive than a turn 4 six drop is a turn 3 six drop.
Sword of Feast and Famine: 8/10
If black could kill artifacts this would be less of a problem, but having your whole deck blanked by a single card you can't possibly answer is the worst feeling. I'm a little more friendly with
sword of light and shadow, since it requires a lot more work to be good. (Much like
sun titan)
Elspeth, Knight Errant: 7/10
I'd be a lot more okay with her if the
Angelic Blessing ability was a minus ability. As it is, doing what she does while only becoming harder to kill makes it play badly.
Umezawa's Jitte: 7/10
While still an insane card, Jitte's been a little more manageable with all the sac outlets to stop it from getting counters. I'm still a little worried though.
Maze of Ith: 7/10
Best described as a removal spell which can only act defensively, and synergises with other removal spells.
Sword of War and Peace: 7/10
Better protections than F&F, but both colors can deal with it and the trigger is less crippling.
Mother of Runes: 6/10
Sometimes she just blanks half your opponent's deck, but she can be manipulated into being vulnerable, like attacking with a dude to force her to give something protection, then using a killspell. Could be a candidate for removal though.
Batterskull: 6/10
I'll admit, I've died to beatdown with this card in play. I think a lot of the aggro decks in my cube go underneath it, or have artifact answers/chump blockers.
Also it has been stoneforged in literally 0 times
I think I might have to go achievement hunting
Jace, the Mind Sculptor: 5/10
I'm amazed, but he hasn't been that oppressive in my games. He does the most work here essentially being unsummon with rebound. Maybe we're just doing it wrong.
Trechery: 5/10
Control magic is insane, but it's becoming worse and worse with the times. It's a strong card, but I don't it's oppressively so.
Balance: 4/10
For balance to be good you need to be a creatureless (ish) deck which dumps it's hand, which usually involves playing lots of artifact mana. Artifact mana which is not currently in those proportions in my cube!
Balance as Wrath, Discard 3 cards is probably a good and fair wrath, it doesn't have the insane card and tempo advantage that it's brethren do.
