This countess is the highest burst damage I have found, I experimented over and over again in practice tool (both with custom new builds and those posted here), and this one's combination of Magical Power + Mana Scaling + Magical Penetration just makes the damage output extremely high.

omeda.city does not take the passive from Azure Core and Oblivion Crown into the total ability power:

NUMBERS

Total Magical Power at full build: 609

You will almost always be above 50% mana at mid and late game so,
Total Magical Penetration: 39.6

Your Q + RMB + E combo will one shot squishy heroes, and here's the best part

Countess ult is normally used after the combo. At this point, even a tank would be shredded and below 40% health without even using ult, specially since you have Magical Penetration in the build.
This means that Wraith Leggings passive would proc and your ult would do 20% more damage to the enemy.

In other words, you can very likely kill a tank with one combo or leave them one-shot for anyone to finish.

IN THE RARE CASE that the enemy tanks are full Magical Armor, and somehow Q + RMB + E combo doesn't get them below 40%,
your ult would be enough to lower them below 40%, at that point just one more E+RMB and they are dead.

NOTES

• This is not taking into consideration Combustion's and Crown's passive that enhance the next instance of ability, which normally proc together when throwing an E or Q at the start of the combo and demolishing the enemy
• Yes, people will argue "Oh but Azure takes too long to be useful".
  1. You don't build Azure first, leave it at Alchemical Rod and only finish Azure as third item.
  2. If you are even remotely good at farming and rotating, you will gain the stacks incredibly easy on top of snowballing with kills.
  3. You are getting a WHOPING 128 bonus Magical Power from the Azure Core's Arcane Magic ALONE.
  4. People have the wrong idea that Azure Core builds take too long to come online. This is somewhat wrong. You're going to be strong enough by the time that you get Wraith Leggings. On top of that the first two items + Trinket give you Max Mana. Even if you aren't at full stacks, the Azure Core's Arcane Magic will be very useful.

Many players believe that skipping Azure Core sacrifices late-game scaling for early and mid-game burst. However, I think that's not the case. With how relatively easy it is to obtain the stacks, and the Max Mana boost from your first two items and Trinket, you're actually just sacrificing mid and late-game scaling for an early game burst.

COMPARISON

I will compare this build to the most liked current build at omeda.city, solely because I am assuming that if it's the most liked then it's the general consensus that this is the best build and therefore this build is the most used.
I'll refer to that build, as CBuild (Common Build) and this build as MBuild (My Build).

CBuild - Wraith Leggings, Tainted Sceper, Oblivion Crown, Caustica and Megacosm with Epoch Trinket

CBuild Total Magical Power: 500 (Gained Magical Power bonus from Oblivion Crown)
MBuild Total Magical Power: 609 (Gained Magical Power bonus from Oblivion Crown and Azure Core)

CBuilds Total Magical Pen: 23 + (20%) = 27.6
Mbuild Total Magical Pen: 33 + (20%) = 39.6

TRINKET

In CBuild, the Epoch trinket is the go-to choice, but in my opinion, it's not the best fit for her kit. Countess is incredibly mobile thanks to her Q, and in team fights, you want to be constantly on the move, positioning yourself for the perfect RMB and E angles. With her passive, hitting multiple targets will also heal you, making Soulbearer and Tempest the superior trinket options. And since MBuild scales with Max Mana, Soulbearer is undoubtedly the top choice in this scenario.

IN CONCLUSION

In my opinion, this build is a hidden gem that often goes overlooked, but it has the potential to carry the entire team. Even if you're losing by a significant margin, the impressive late-game scaling allows for an incredible comeback. As a skilled Countess player, you'll be leaving your enemies with broken ankles, and any squishy left alone is essentially a death sentence. Don't underestimate the power of this build!