Sounds like you didn't try a high-Int barbarian with the like of Hours of St. Rumbaldt's Greatsword proning nearly entire battlefields with every swing
Or the St. Ydwen's Redeemer greatsword slaying Vessels left, right and center. There was plenty of OP stuff in PoE1.
But PoE2 has its share also. Of course, the addition of multiclasses added an an extra layer of synergies between the classes.
One of the most broken things are various combos/synergies (ab)using dual aoe blunderbuss mortar weapons - Serafen comes with the Hand Mortar and you get Fire in the Hole during his questline (that can be finished very early). Most special abilities have aoe effects with them, which is very potent by itself, particularly when you consider there are 2 aoe chances to apply per char - actually 3, because you can enchant Fire in the Hole with Chainshot bouncing effect (stuff like Monk's Stunning Surge, Rogue Blinding/Crippling/Devastating/Toxic, Paladin Flames of Devotion). Also stuff like Combusting Wounds or Avenging Storm spell (available on a helmet) triggers effect per hit per target, so... Things start to really get broken if you use the mortars with abilities that are aoe themselves.
Most come late and are available to single classes (Monk Whispers of the Wind, Barbarian Heart of Fury, Rogue Vanishing Strikes; maybe even Ranger Whirling Strikes and Fighter Clear the Path with mortar/melee weapon combo), but 2x aoe weapon x aoe abilities = beastly carnage.
Then you can use some tricks like monk's Flagellant's Path to skip the reload on firearms. Charge across the enemies, damaging everyone in your path, full aoe attack your target and then instantly fire another dual aoe salvo.
Another interaction I really like is connected to the Grave Calling Saber. It can be gotten near the beginning of the game. And it can be enchanted to create a party-friendly Chillfog (aoe blinding and cold-damaging over-time effect) whenever you kill a vessel with it. Another enchantment lets it Paralyze enemies after accumulating 10 damage stacks in a fight. Thing is, those don't have to be melee hits. Aoe ticks from Chillfogs created by the saber also work. So accumulating 10 is trivial and takes no time this way. More importantly, once the weapon has those stacks, the Chillfogs it created also Paralyze enemies with every tick. Even though the damage per tick and Penetration on Chillfog isn't very good, once you stack several atop each other, they can prove to be quite deadly.
Of course, even though they are fairly common, there aren't vessels (mainly undead and constructs) present in every encounter. But incidentally a Chanter can summon 3 Skeletons with his 1st level invocation (6 in case of Beckoner). And another high level Chant lets him summon a skeleton passively every 6 seconds (or 3 in case of a Troubadour's Brisk Recitation).
So if you have a Chanter in your party, you never run out of "fuel" for Grave Calling.
I like to use a Cipher/Helwalker as a Grave Calling wielder (with a blunderbuss offhand), as Helwalker might and Thunderous Blows amplify Chillfog damage and Penetration, monk Duality Int boost greatly increases aoe radius and paralysis effect duration. On the other hand, since the Chillfogs are still flagged as damage originating from weapons, the aoe damage also fills Cipher Focus, almost "passively".
One possible, albeit slightly risky, twist to these tactics is to use a frenzied Berserker for Grave Calling vessel kills. Preferably with a Beckoner for double amount of skeletons. Don't remove the Berserker's Confusion and the Paralyzing Chillfogs will no longer be party-friendly. On the other hand, even one Chillfog will quickly consume the rest of the summoned skeletons, rapidly stacking lots of Chillfog effects. Rapidly melting encounters (and party members unfortunate enough to be inside), as well as your graphics card...
