Beings with the power levels to destroy entire worlds pop up with some frequency in comic books. It's kind of the nature of the game. But there are always some characters who stand out as incredible threats, and for more reasons than just their power levels.
Watchmen's Dr. Manhattan and Marvel's Thanos are very different characters, each one of them incredibly dangerous in their own ways. If the two ever faced off, the results would rip the fabric of reality itself completely apart – but there has to be a winner, even though a heavy price would be paid by whoever came out victorious.
11 THANOS: A Titanian Eternal
Since day one, Thanos' birthright set him apart from even his closest peers. Born on Saturn's moon, Titan, this colony was an offshoot of the Eternals living on Earth. That gave Thanos incredible abilities, but wasn't the end of what made him different.
Thanos was born with Deviant Syndrome, which essentially made him a mutant within his own race. Because of this syndrome, Thanos was purple and much larger than other Titans. These differences resulted in him suffering significant abuse, which set him on the road to being becoming the Mad Titan.
10 DR. MANHATTAN: Omnipowerful
There's powerful, then there's Dr. Manhattan levels of power. Jon Osterman used to be a nuclear scientist working on experiments for the US government, but an accident gave him unbelievable powers, something the rest of the Watchmen did not have. Turning completely blue, Osterman became Dr. Manhattan. His power levels were of the charts, which made him virtually unstoppable. He knows everything, and can accomplish virtually anything.
9 THANOS: Super Genius
When it comes to being a super genius, Marvel has more than a few heroes and villains who fit in that category. Reed Richards, Doctor Doom, and others use their vast intellects in a variety of ways, many of which don't pan out well for the world.
Thanos is a unique super genius for one big reason. His intellect is informed by an understanding of the universe's inner workings that few can match. Things that others theorize about, Thanos knows as a fact. And that knowledge makes him powerful.
8 DR. MANHATTAN: Can Manipulate Reality
The most overt uses of Dr. Manhattan's powers are as impressive as they are devastating. He has been known to grow to enormous sizes, then generate destructive blasts to destroy entire armies. He once disintegrated Rorschach with ease.
But the most frightening thing that Dr. Manhattan is capable of is his manipulation of reality, on both a macro and micro scale. Honestly, it would be smart for anyone around him to not fully trust what they perceive as real. The world is quite literally what Dr. Manhattan makes of it.
7 THANOS: Death Has His Back
Love can be a powerful motivator. In the MCU, Thanos was driven by a desire to bring balance to the entire universe. In the comics, he snapped half of life out of existence because he was quite literally in love with Lady Death. While she hasn't always approved of everything he's done – and is disappointed by his frequent failures – Lady Death does have some feelings for the Mad Titan. The fact that he has Lady Death helping him, even if it's only a manipulation, still gives Thanos an edge.
6 DR. MANHATTAN: Utterly Emotionless
If it seems like Dr. Manhattan doesn't care, it's because he quite literally does not care. Jon Osterman's transformation into Dr. Manhattan cost him any emotional connection to the world around him. That's why he ended up moving to Mars at the end of the original Watchmen.
This level of disconnect from the world makes Dr. Manhattan incredibly dangerous, as Rorschach found out first hand. The combination of having virtually unlimited power and not caring about the impact of using those abilities means he's literally capable of anything.
5 THANOS: Brilliant Strategist
Thanos has a lot of options at his disposal. He's a powerful, skilled fighter with an impressive array of cosmic weapons at his disposal. Thanos has a seemingly endless range of abilities that give him advantage against almost any enemy.
But his greatest advantage is, without question, his mind. Thanos is an virtually unparalleled strategist, capable of developing and implement impressive plans. That being said, he has a tendency to subconsciously build ways to defeat him into those same strategies.
4 DR. MANHATTAN: Absolute Understanding Of Everything
In addition using his abilities such to blow things up and rewrite reality, Dr. Manhattan has another incredible edge that goes well beyond simply being intelligent. Jon Osterman was a genius before he got his powers and becoming Dr. Manhattan made him cosmically aware on a completely different level. For all intents and purposes, he has an absolute understanding of all aspects of time and space.
3 THANOS: Huge Range Of Abilities
What makes Thanos so impressive is that he has such a diverse range of powers. Being an Eternal automatically gives him a level of immortality, meaning that he doesn't really age and is immune to almost all diseases and toxins.
But he's also superhumanly strong, seemingly without any upper limit. That's in addition to enhanced speed, reflexes, senses, durability and more. Thanos can manipulate energy, is somewhat telepathic, and is even trained to an extent in the mystic arts.
2 DR. MANHATTAN: Invulnerable To All Attacks
There's another side to Dr. Manhattan's impressive, overwhelming powers. The man is invulnerable to any and all attacks. While there might be a way to take him down, it's honestly hard to imagine what that would be.
Dr. Manhattan is basically a god. He has the power to do whatever and there is seemingly nothing capable of stopping him. This understandably scares everyone on his Earth.
1 WHO WOULD WIN?
Thanos should not be underestimated in any fight, but unless he's wearing the Infinity Gauntlet or has a literal sack full of Cosmic Cubes, he isn't beating Dr. Manhattan. Without the aid of any super powerful objects, or pants for that matter, Dr. Manhattan could easily snap Thanos out of existence.
The Mad Titan has devoted his life to finding ways to gain the power Dr. Manhattan already has. Simply put, Thanos would need the most brilliant strategy he has ever devised in addition to every cosmic weapon he has ever wielded to even stand a chance of surviving this encounter.