Ok so this is my list of who I think is the strongest in Ben 10. I'll also provide reasoning behind my ranking for the first few powerful ones.
This one is like obvious I mean. They warp reality on a universal scale eg. art style changes. Can tank a universe busting attack with no scratch. Can destroy and create a universe with a single thought and according to writer's statement can destroy the multiverse with 6 thoughts. That's a lot of power. They do have some weaknesses that are mentioned in their wiki.
(I'm also putting the wielder of Omnitrix in this section because the wielder of Omnitrix is strongest when transformed into a celestial sapien.)
#2 Contemelia
The creators of the universe and Anihilaarg. They're 5th dimensional beings and can't be seen in their usual form by most beings, instead they see something or someone they really like or respect. Not much is known about them but since they have technology to create and destroy universes. I'm ranking them as number 2.
He can go anywhere in space and time with some limits. He has a high cosmic awareness and is also knowledgable on the different possible futures and other parallel universes. He also has control over time being able to slow it down or stop it.
#4 Maltruant
I'm mentioning Maltruant specifically instead of everyone in Maltruant's race. Because the only other we have seen is clockwork and he didn't seem that powerful to me.
Maltruant has shown immense power over time. Being able to slow it down and gaining super speed as a result or stop it completely for the target thereby freezing them in time. He managed to defeat Atomic-X AND Gwen with all the Charms of Bezel combined. He can also travel to parallel universes.
He however doesn't have the cosmic awareness or knowledge of things like Paradox does as far as I can tell. He also needs a time beast to travel very far into time.
#5 Naljian
Before I can talk about number 5 I'd like to say that technology in Ben 10 is very highly superior. There are weapons that can even defeat beings like Diagon and Anodites. Azmuth even created the Omnitrix which can give the user the powers of a CelestialSapien.
And among all the other high tech races in Ben 10 Naljian ranks highest among them. Even their children's toys are a higher level of technology than the omnitrix which is the highest level of technology in the known universe. They also seem to possess powers like telekinesis and cross dimensional travel.
#5 Smart High tech races
I'm also putting the other high tech races here like Galvan with Azmuth ranking even above, Cerebrocrustacean, Weapon Masters of Techadon's species, Jury Rigg's Species and others here because while they themselves aren't that strong they're able to create weapons/devices that can beat very powerful opponents
#6 Diagon
Diagon is an extremely powerful being of pure energy with multitude of abilities. His heart is said to be able to reshape the entire universe. He could instantly mind control the entire earth population.
He does have a weakness to energy siphoning devices with no upperlimit.
Vilgax (Diagon Absorbed) also ranks number 6.
#7 Alpha Rune wielder
The alpha rune is a very powerful magical artifact. Said to give it's user control over magic itself. It's user can mind control all the inhabitants of ledger domain. Instantly drain the souls of anyone they want and even beat an anodite instantly by draining their mana.
Charmcaster gets special mention here as the only character that actually managed to kill Ben. And Gwen(twice but the first one's different) and Kevin. All because she wielded the alpha rune.
#7 Malware
Malware was one of the strongest villains Ben faced. He could absorb technology and upgrade himself. And due to how advanced and incredible amounts of technology exist in the Ben 10 universe. He's potential is immeasurable.
He has a weakness to Conductoids (Feedback's species)
Malgax(Vilgax with Malware remains) is also in this slot.
#8 Anodites.
Anodites are beings made of mana i.e life energy and are able to control it to achieve many things. They are said to have enough power to keep the super high tech synthroid planet powered for years. They can also warp reality. Verdona changed Ben back to normal when he was transformed into spider monkey by snapping her finger. They have incredible speed being able to fly between earth and their home planet in a very short amount of time, when controlled by Dagon Gwen could run at incredible speeds too. Gwen when transformed into her anodite form blasted a high breed miles away with a single energy blast. She also casually flicked away Humangasaur and changed him back to Ben when controlled by Diagon. They can teleport and create portals. They can absorb Mana from organic life to further augment their powers. They can also time travel through magic as well as stop time.
Despite all these powers there is mana dampening technology that can suppress most of their powers.
#8 Magic users (Sorcerors)
I'm putting the magic users in number 8 for the same reason I put the smart high tech races at number 5. While they themselves aren't that strong they can prepare a spell or enchanted rune or siphon power from stronger power sources that can take down opponents much stronger than themselves.
Bezel gets special mention as one of the most powerful and ingenious sorceror. He created the charms of bezel. He's like the Azmuth of sorcerors.
#8 Atomic-X
Atomic-X is the second strongest transformation of Ben. The fusion of Atomix and Alien X is really powerful. Has very high super strength. Being able to knock Vilgax with a flick of the finger. Creating multiple small black holes and light fission. He can also fly and use powerful energy blasts. Exact extent and limit of their power is not known but they can't handle time attacks.
Although the wiki states they're omnipotent, feats wise he seems far from it to me. And as I'm going through feats in this list he's in number 7 instead of number 2 or 3.
Also any other possible fusions of Alien-X + any other alien will also be in this spot except for maybe Clockwork+Alien-x (Clock-X) and smart race beings like Greymatter and Brainstorm + Alien X.
Way big itself is a very powerful being. Ultimate way big is even stronger. Being even larger than Waay big with even higher amount of cosmic power.
#10 Atomix's species
Atomix and his species are one of the strongest in Ben 10. Atomix easily defeats Ultimate humangasaur with a single punch and takes out ultimate spider monkey with a single blast. He can also fly and has super speed.
They're a very formidable race. They have considerable strength even for their size as Waybig defeated Vilgax even when he was as large as him. They can also shoot beams of cosmic energy.
Despite all these powers they have been defeated by characters like Argit or through electrocution from the dimensional gate. Their head fin is also a weak spot.
#12 Ma Vreedle
Not really sure if I should put her here.
She's one of the most feared criminals in the universe to the point that even Vilgax runs away from her. Her strength is extremely high. She once defeated the entire plumber HQ.
She also possesses high intellect being able to create high tech weapons but she still acts without thinking like most Vreedles.
#13 Ultimate Kevin Levin
He is the strongest mutation Kevin had so far. He has absorbed several superpowers and is able to use them quite well. He can fly in hyperspace, survive in space, become intangible or fluid like Goop. Pyrokinesis, electrokinesis, Chlorokinesis, Crystalokinesis, Hydrokinesis, Cryokinesis, Electrokinesis, Magnetism, Super sonic scream, Super speed, Super strength, Super durability, regeneration(Goop and swampfire) etc.
Despite all his powers he didn't have any defense against sound based attacks which allowed Ultimate Echo echo to take him out fairly quick.
#14 The wielder of Charms of Bezel
The charms of bezel gives it's wielder
Charm of Luck and Probability :-Gives the user supernatural good luck and bad luck to opponents.
Charm of Resurrection :-Gives the user the ability to heal from injury and even come back from the dead.
Charm of Pyrokinesis:- Allows you to generate fire and control it.
Charm of Electrokinesis:-Allows you to generate electricity and control any kind of electricity.
Charm of Telekinesis:-Gives the wielder the ability to use telekinesis on multiple people including themselves and objects.
KeyStone of bezel giving a x10 multiplier to the user's natural abilities and abilities and the other charms.
#15 Ultimate Aggregor
Has the power of Terraspin, Water Hazzard, Armodrillo and NRG. The combination of these powers made him a very formidable foe.
His powers include
Andreas' enhanced strength, durability, and shockwaves.
Bivalvan's invulnerability and hydrokinesis.
Galapagus's immunity to mana and aerokinesis.
P'andor's radioactive energy projection, heat generation, and immunity to heat/cold.
Ra'ad's electricity manipulation, mind reading, flight, and the ability to breathe underwater.
#16 Vilgax AF
This Vilgax absorbed the champions from 10 different planets.
His strength was now enhanced much more.
He could fly in hyperspace as well as survive in space.
He can shoot Optic blasts that can change directions and catch upto Helen who can run faster than the eye can see.
He also had the Shield of Ziegel to generate a powerful personal force field.
He had the Ruby Ray of Ulo which he could use as a laser projector.
His gauntlet could be used as a taser to hurt even intangible beings.
Vilgax had a Sword that was very sharp and could shoot energy, turn into a flaming sword or a crystal sword to refract energy.
#16 Vilgax as Dagon's minion
This is the form Vilgax gained while working for Diagon. Aside from his usual strength he could shoot energy beams and had the strength to keep up with Sir George and Eatle at the same time.
#16 Wielder of Ascalon
Ascalon taps into the forces of the universe itself as the source of its power. Misuse of that power can destroy an entire planet.
These are the powers it grants it's wielder
Ascalon is incredibly durable, being able to survive an exploding planet.
Ascalon's blade is powerful enough to cut through metal and block or reflect energy blasts.
Projecting energy blasts.
Channeling its power through its armor to project shockwaves.
Opening portals for transport.
When the users spins it, Ascalon can create whirlwinds that are strong enough to send Humungousaur flying.
The sword is linked to the one who wields it, allowing them to manipulate and control the sword even when they aren't holding it.
Erecting force fields.
Absorbing the powers of its opponents.
Changing the user of the Omnitrix/Ultimatrix back to normal.
#16 Ultimate Humangasaur
#16 Ultimate Echo Echo
#16 Conductoid (Feedback's species)
#17 Kevin 11,000
#18 Zs'Skayr
#18 Vilgax OS
#19 Tetraman
#19 Petrosapien
#19 Pyrosian
#19 Ultimate Spider Monkey
#19 Kevin 11 OS
#19 Kicking Hawk
#19 Humangasaur
#19 Big Chill
#19 Rath
#19 Exo Skull
#19 Canon ball
#19 Gravattack
#19 Lodestar
#20 Syphon
#20 Julie with Ship's armour
#20 Clyde 5 Sincotrix
#20 Xylene
#20 Rojo OS
#20 XLR8
#20 Fastrack
#20 Six six, Seven Seven and Eight eight
#21 Rook Blonko (proto suit)
#22 Zombozzo
#22 Chamalien
#22 Vulcanis
I'm aware that there are some aliens and characters I haven't ranked. There's just so many. You can give me your suggestions on where they should rank. Also let me know what you think of this list.