For many, Thanos is the best villain we've had in decades of cheesy and generic movie supervillains. The purple alien with the fixation for a golden Mardi Gras gauntlet has known loss and suffering better than most heroes and is willing to break the cycle of the universe. He's perfectly balanced, as all things should be.
Complex and powerful as he may be, Thanos isn't the be-all and end-all when it comes to Marvel villains. This statement applies both for comic books and the Marvel Cinematic Universe (MCU).
There exists a lineup of evil beings (both in DC and Marvel) which are certainly more powerful than the Mad Titan, even if they don't look the part. Here are 10 of them that Thanos would do well to never underestimate.
10 Mr. Mxyzptlk
First off in this list, a certified troublemaker in the DC universe who's way beyond our comprehension (or pronunciation capabilities): Mr. Mxyzptlk. He's a being of the 5th dimension, something we third-dimensioners will find difficult to understand; so let's just say Mxyzptlk has the capability to snap the universe out of existence.
Being a fifth-dimension imp, Mr. Mxyzptlk has full control over the reality of three-dimensional universes. As such, he can bend anything to his will in our plane of existence. Thanos, on the other hand, needs his Mad Titan starter kit in order to do that.
9 Dormammu
While Dormammu might appear like a giant blob of grease floating in space, he's something that will probably give Thanos a good and hard fight. As you can recall in the MCU, Dormammu is more than a giant physical being, meaning he's not someone you can fight conventionally. Even Doctor Strange couldn't find a way to defeat him so he just annoyed Dormammu into submission.
For some reason, however, and despite Dormammu's might, he doesn't seem to be as ambitious as Thanos. Also, Dormammu's certainly not as smart as Thanos is but make no mistake, in his own Dark Dimension, Dormammu is unparalleled and has even dominated his own dimension/universe, something the Mad Titan couldn't do.
8 Lobo
Initially a humorous poke at the dark and edgy superheroes of the 1990s, Lobo has become a favorite of many DC fans. This was all thanks to his blunt and optimistically crass behavior; it lends much to his comic-relief qualities. Despite being made in the spirit of fun, Lobo is no slouch. He can defeat a regular Kryptonian if he wanted and if he's serious (which is seldom).
Regardless, anyone that can go toe-to-toe with Superman (without using kryptonite or a red sun) is without a doubt, more powerful than Thanos. To top it off, Lobo cannot be killed. He's functionally immortal yet super strong at the same time. He's not the first comic-relief that's more powerful than Thanos, though.
7 Hela
Even with an Infinity Gauntlet, Thor with his Stormbreaker is capable of defeating Thanos (at least in the MCU). Surprisingly, Thor isn't even the most powerful Asgardian; his father, Odin, is leagues above him. Meanwhile, another Asgardian, Hela (the goddess of death in Asgard), is also stronger than Thor.
Hela was powerful enough to defeat Thor and even take over Asgard on her own. As it happens, Thanos (in the comic books) has a certain weakness for women of death or those who have control over death. We have reason to believe he's not going to fight her... literally.
6 Reverse-Flash
Reverse-Flash or Professor Zoom is one of the most dangerous and formidable enemies of the Flash in DC and he's ruined the DC universe more than once. For those who aren't familiar with their powers, a Flash can tap into the Speed Force which lets them do all sorts of things like being faster than light or time travelling.
That's essentially what makes Reverse-Flash more powerful than Thanos. Reverse-Flash can easily go back to Thanos' past and undo a lot of things or even erase the Mad Titan from existence. By comparison, Thanos would need the Time Stone/Gem in order to do this.
5 Surtur
Hela is assumingly more powerful than Thanos but it turns out, there's someone out there in Asgard a lot more powerful than her: Surtur. He's MCU's bringer of Ragnarok, someone Thor had to unleash in order to defeat Hela.
As expected, Surtur was physically powerful enough to destroy a plane of existence (Asgard) and defeat Hela. It's safe to assume he's more powerful than Thanos and would still give him problems even with the Infinity Stones/Gems. Heck, even Thor could do it with Stormbreaker.
4 Kang
Kang the Conqueror is hard to fathom or explain; just know that he's a villain from the far future of Earth and is also a descendant of the polymath Reed Richards (Mr. Fantastic). He chose to travel back in time to the past in order to conquer the poor hapless homo sapiens who have yet to evolve to his levels.
What makes Kang more inherently powerful than Thanos is similar to Reverse-Flash's, time travel. Kang isn't someone who would face Thanos without any chance or master plan for success. That means he's more likely to travel to Thanos' past in order to defeat Thanos in the future; a little planning goes a long way, especially if you couple it with time travel.
3 Dr. Manhattan
Rounding up the Infinity Stones/Gems and becoming omnipotent is Thanos' life's work. Needless to say, he took years to achieve that goal and the path was paved with a massive pile of bodies. It's a tremendous effort but there are those in the DC universe, such as Dr. Manhattan, who can do what Thanos does with the Infinity Gauntlet without putting any work at all.
In fact, Dr. Manhattan simply got his powers due to a nuclear accident. Since then, he has been playing god in several DC timelines and universes. Let's just say Dr. Manhattan is not someone Thanos can kill even with all the Infinity Stones/Gems and the Gauntlet. Adversely, Dr. Manhattan can end Thanos before he's even born.
2 Molecule Man
Molecule Man is pretty much the resident Dr. Manhattan of the Marvel universe. He's regarded as the most powerful being in the universe he's in, which means he can easily shape things to his will. In that regard, he's pretty much an Infinity Gauntlet with stones/gems in his own right.
Even if Thanos had the Infinity apparel, Molecule Man would still be a match for him. To that end, there's no doubt Molecule Man is more powerful than a regular Thanos. In fact, someone even realized the extent of Molecule Man's power and stole it for his own use...
1 God Emperor Doom
God Emperor Doom is the ultimate version of Victor von Doom and the end result of all our fanboyism (or fangirlism) for the Fantastic Four supervillain. Doom stole Molecule Man's powers and used it to reshape the multiverse to his own preference and goals. For that time in the Secret Wars arc, Doom became the most powerful being in Marvel.
Thanos was uppity enough to face God Emperor Doom after his transformation and guess what happened? He reduced the Mad Titan to nothing but a skull attached to a spine. Doom even defeated the Black Panther wielding the Infinity Gauntlet with all the stones as a testament to his power. Of course, Marvel couldn't let Doom stay this way and he had to be defeated.