Shock: 12.5% increased DMG per stack Debuff and you can have a total of 4 . Also I discovered in game that it works from hirlings and if you have both characters with shock, it technicallly acts like 8× stack Debuff instead of 4 for doubled DMG. Instead of 50% increased DMG, 100% increased DMG. This is very good in 2v2s, hirling floor climbing builds of all types and even farming builds. Every build ever and stack Debuff works great with other elements. Fire + shock, ice + shock, poison + shock, you name it! Shock is ultimately in PvP as well.
It also works extremely well with the Electrocution set affix, one that you get from Amethyst - just like crushing flames, Demonic and angelic , the affixes you wouldn't expect to be in Amethyst.
Obviously with elemental crit of paralyze to stun enemies and high voltage of 100+% to increase DMG to stunned/paralyzed enemies by 100% (even if the DMG is another element).
Arcane: Well it doesn't have its own effect but Ascendant set gives it a Debuff effect of reducing enemies other resists by 62.5% at (5) which is like equipping 62.5% weaken for free but it guarantees a loss in enemy resists. Arcanist set affix gives it a buff where for enemy weakened and under arcane Debuff (whichever works it comes first), your ED% of any type gets increased by 20% . Ascendant also has a prismatic effect of using other elements to capitalise on the Debuff of reducing resists by 62.5% and it is available in all classes , unlike how wizard is the only one with prismatic talent.
There is the orbit skill that can be increased by Celestial and this happens on elemental crit but the aim isn't that work and good. It suffers the Whirling Blade situation because both have the inaccuracies. Although arcane orb can be spammed , especially with coat skill and it could deal decent damage with enough Celestial and DMG. Of course if would be better buffed via debuffer hirling as well and/or other elements.
Fire: It has a dot of 40% per second but that can be increased to 80% per second with Inferno set affix because it speeds up dot time. Theres also Blistering which gives bleed that deals 100%+ of fire dot which can be increased via Vampiric Touch by 125% and that blistering can also be affected by Inferno when enemy is on fire. Not just blistering but the bleed that comes from 10%+ chance also can be affected by Inferno and is just as deadly, also being increased by Vampiric Touch.
There is the crushing flames set though that drastically increases the DMG of crushing blow as long as enemies are Immolated and increases immolation DMG by over 125% or so. Immolate already does 100% of the fire dot but with immolation at 100%, it does over 200% of the fire dot or more . Although crushing flames is probably so good that you'll defeat them quick enough even with lower DMG than you'd normally have, but having overkill DMG is great with any form of crushing blow since crushing blow doesn't defeat enemies on its own, constant hitting and DMG also helps.
Ice: It has a simple Debuff of reducing enemy move speed by 50% and even reduce their DMG by 25% . This is also good in PvP to some extent with the damage reduction and move speed loss because combined with slow trophy and your enemy stops moving completely or extremely slower. On enemies in general, it's very good.
Now it's seems least damaging until you have freeze to freeze the enemies, Frostbiting to increase DMG by 100%+ on said frozen enemies and the Brutal Frozen set to make them explode when death. The enemies deal DMG exponentially when dead and frozen explosions and the spike damage can increase to ridiculous levels. That's why we see Trillions of DMG or more. Goes great with Demonic set , Relentless Talent , Crit DMG , deadly strikes + Brutal mythic , Double Strike to spam it twice sometimes , stack Debuff enemies , arcane debuffed enemies, weaken 100%+ , etc. It's just too good to not use.
In PvP, the freeze helps as well but the only issue is that the elemental Crits are not worth the slots when opponent uses flawless victory, unless you use the coat skill from rogue for eg. That's for ice and shock in arena. Also stun on players is 1 or less seconds. Freeze and Frostbiting in arena can work somewhat well but the fact if stopping opponent is more than enough in some cases.
There is also Freeze resist (formerly known as Freeze immune) which makes players resistant to freeze just like how stun resist makes players more resistant to stuns (though the resists are not really needed that much).
Poison: Lastly poison. This element used to be the most OP element until recently and now ice is the most OP and fire comes in at the place when you have crushing flames.
Poison dot deals 12.5% DMG per second for 8 seconds which doesn't sound like much but then there's other factors. Poison cloud increases the poison dot DMG by a huge 400% per second and Blight 100%+ can increase this to double the amount aka 800% or 12.5%×8=100%+or 12.5%×9. If you then use Plagued set affix, you can group a bunch of enemies together and increase dot DMG by 125% per enemy and this gets multiplied by Toxic poison clouds and Blight for huge DMG. It's actually really powerful at crowd control as well, second to ice at that.
That high poison dot and cloud DMG goes on for about 8+ seconds and with the cloud, it makes that insane dot damage carry over to however many enemies caught into the cloud as if it were exponential. The poison cloud can buff it more than once but no more than that. However it used to do so exponentially which meant theoretical infinite DMG but that's not the case anymore.
Also there was once a time when poison increased crit DMG hugely but that's not anymore and poison cloud was once able to deal the crushing blow DMG and exponentially growing in DPS . Not anymore though.
Druidic can increase poison dot by every multiplier per 4000 Regen. At 4000 Regen, your poison dot/cloud gets buffed by ×2 which is already so good. At max cap which means both the cap of mana and HP Regen, the poison DMG can be so damn high. It costs a lot of slots though .
Even nowadays, you could still get trillion DMG poison if you really wanted to and billions of DMG build in general. Poison cloud builds with plagued , blight and maybe druidic also goes very greatly with affixes like High Voltage in some cases , the stack Debuff from said hirling or main character and even arcane Debuff. Also the Frostbiting affix. Coat skill was also ridiculously OP with poison until the toxic can only spike once after it's already striked but even once is huge.
Combination builds can get the best of world's and sometimes be most damaging builds ever with two elements in synergy, but a one element build can harness their full potential. Poison is one such case for example