Pretty much everything can be countered nowadays. There are so many attacking combos that any team can be defeated. But if i had to build a defensive team i would have these heroes in mind:
Mad hatter (either disc), with max red skill
Megara (sh), because of the link she gives
Angel (st), because sometimes if she protects the right hero, you can turn the fight around
Kermit (go), because of the invincibility
Poca (ki), with max red skill because she enables numerous fast combos
And then, there are some “unstable” options like, shego,ron,bolt, disgust, mr Big. I call them unstable because you can’t predict with big certainty how they will react in battle. I mentioned angel before. If angel protects one of the heroes mentioned above, they can turn the fight around. These toons might also have high evasion. I like unstable heroes in defense because the attacking enemy might fail on it.
Finally, and probably most important, the mods have big impact on the defence you are building. Teams that have heroes with attack speed mods can be more dangerous. Some upgrades on lvl based skills, like Minnie’s purple skill can help you ensure that this skill won’t get evaded.
But, still, there will probably be three teams to counter someone’s full defense. All the above are just my thoughts on how to try and not get swept on one hit. Surviving once or twice during an attack is a huge win, during close wars!