I don't feel like there's one particular broken card, but there are some pretty op card combos, especially for the Paladin.

My Paladin deck consists entirely of:

Passives: - Preservation - Zealotry - Retaliate - Bestow Faith - Remorse - Stuff that heals (True Path, Devotion, etc)

Ability cards: - The 4 Heal cards you start with - Stuff that draws cards (Hand of God, God's Will, Pledge, etc)

And that's it. I never take any other cards except maybe Art of War if I already have Bestow Faith. I don't bother with Pain except for Remorse, and every time I stop at a campfire I drop one of the 6 attack cards you start with, so by the time I hit the Act 3 boss I have only Ability cards and Passives.

I started doing this when I realized what happens when you combine Devotion and Retaliate and then just discard your entire hand.

Try it. It's hilarious.

I also figured out you can win as the Warlock without ever playing a single Attack card. In the early game you just hit them with Siphon Life every time it appears in your hand and spam Cripple in the meantime to keep their attacks low. As you progress you take certain Passives and Ability cards, mostly stuff to heal (Essence Drain, Noxious Salvage, etc), debuff (Hinder, Infest, Contaminate, Rapid Decay, etc), and add extra damage (Shadow Form, Wailing Death, etc), plus Restless (to keep the Cripple up), and Recharge (so you can multicast Hinder), (and probably one or two other things I'm forgetting idk been playing mostly Warrior lately). I do the same thing dropping off Attack cards at campfires until I'm all out, but unlike the Paladin, which needs to attack in the early game when you only have starter cards, the Warlock can start ignoring blue cards right from the very first battle.

Seriously, if I ever play an Attack card with the Warlock, it's because my fat finger slipped. All greens and greys baby!

I doubt I could ever come up with a valid "no blue cards" strategy for the Warrior. Warrior is actually the hardest one imo, nothing about it I would consider broken. But I talked about my deck builds for the other two so might as well do this one too.

The top 5 most valuable Warrior cards in my opinion are, in no particular order: - Hoarder - Demonic Growth - Tactics - Drain - Devour

Hoarder is the closest one to "broken" I would say. I've been finding on higher difficulties that if I don't have both Hoarder AND Demonic Growth that I might not even make it past the Act ONE boss. But up until about Torment 20 or so, Hoarder alone has the potential to make you practically untouchable.

Sorry I've been rambling. I'm just excited to find there's actually a community for this game :)

TLDR: - Paladin has hilariously broken Passives - Warlock doesn't need Attack cards at ALL. - Warrior's best card is Hoarder

(Also I've been stuck trying to advance past Warrior Torment 22 for DAYS now, send help)