Thanos showed up for a very small time in the Marvel Cinematic Universe, unlike Marvel Comics, where he has been around for years and years. He hasn't stopped his will to please Lady Death and unless he achieves his target, the Mad Titan is most probably going to continue to wreak havoc across the galaxy.
Despite Thanos being an Eternal (nigh-invulnerable), he isn't a godly being who cannot be defeated. There is a way to get at him, preferably when he doesn't have the support of the Infinity Gauntlet with all the Infinity Stones. The Mad Titan has suffered some bashings over the years and it is going to be difficult to rank them.
10 Mar-Vell — Thanos was willing to die to meet up with Lady Death & sacrificed himself.
The annoying thing for Thanos about losing to Mar-Vell or Captain Marvel wasn't that he lost but that his plan didn't work. This all happened in an alternate universe, where Thanos was willing to die to meet up with Lady Death.
As a result, he sacrificed himself by fighting and dying at the hands of Captain Marvel. Unfortunately, when he came to meet Lady Death, she wasn't pleased with the Mad Titan's lack of loyalty and she left him alone.
9 Adam Warlock — He made Thanos think that the Infinity Gauntlet wasn't everything & took it for himself.
Adam Warlock is one of the most famous bearers of the Infinity Gauntlet and he had to beat Thanos to the Infinity Stones. This wasn't a proper battle between two immensely strong celestial beings but it was more a case of Warlock putting things into Thanos' huge head.
He manipulated the Mad Titan into thinking that the Infinity Gauntlet wasn't everything. And when he won the psychological battle, he took the Infinity Gauntlet for himself, thus fooling the Marvel supervillain.
8 Drax — as the sworn enemy of Thanos, Drax gave him a taste of his own medicine.
Drax is pretty much the sworn enemy of Thanos and unlike the Marvel Cinematic Universe, the Marvel Comics version of Drax the Destroyer isn't as weak. In fact, he is very strong and he was brought into existence keeping the power of the Mad Titan in mind.
This was the time when Thanos teamed up with Annihilus and he had a proper army. But it was nothing for Drax who smashed everyone before giving Thanos a taste of his own medicine.
7 Wolverine — Logan fooled The Mad Titan by going undercover & proceeded to cut off his arm before taking the Infinity Gauntlet.
Wolverine played Thanos like a charm. In one storyline from the What If…? series, Logan goes undercover and acts as an advisor to the Mad Titan. His acting performance is so good that it fools someone with a genius-level intellect.
At the right opportunity, Wolverine uses his claws to cut off Thanos' Infinity Gauntlet's wearing arm before pounding upon him, with the help of his teammates. It was quite a moment for Logan who brought down a celestial being with the use of his brain.
6 Groot — Groot shouldn't be angered, but Thanos learned it the hard way.
Groot shouldn't be angered, much like The Hulk. Unfortunately, Thanos learned it the hard way when his Cosmic Cube-backed powers were stripped off using a cosmic weapon by the Guardians of the Galaxy. Groot began to hammer the Mad Titan.
He looked so mad that it felt like he wouldn't stop until the Guardians of the Galaxy intervened and knocked the Mad Titan out. This, once again, proves how easily Thanos could be beaten if there aren't supernatural cosmic forces backing him.
5 Thane — He wanted to teach his father a proper lesson & trapped Thanos in an inescapable cosmic prison.
Losing to Thane wouldn't have been such a big deal if he wasn't the Mad Titan's son. They shared a very complicated history and Thane was hell-bent on teaching his father a proper lesson.
His superpowers were incredible as well, ranging from altering reality to create inescapable cosmic prisons. He trapped Thanos in one of those prisons before the Mad Titan could even settle. Yes, the Eternal lost to his son quite easily.
4 Ka-Zar — a brilliant hand-to-hand combatant, he punched Thanos until he fell into mountain lava.
Ka-Zar is a nobody in comparison to Thanos. He lives in the wild and is simply a brilliant hand-to-hand combatant with no standout superpowers. Thanos found himself in the place where he resides called the Savage Land and the Mad Titan was caught napping with right hooks after right hooks.
He was being absolutely battered before he fell into the mountain lava. He didn't die, he was just left figuring how he was defeated by a mere mortal with nothing special.
3 Starlord — He used the Cosmic Cube to embarrassingly knock out the Mad Titan.
The Guardians of the Galaxy know Thanos all too well. Obviously, the Gamora factor plays into it but that's not the only thing. Thanos is almost always cooking up evil plans to take over the whole galaxy and in this instance, he was newly resurrected.
Starlord is not someone fans would associate with as the "Thanos beating" kind but he did -- and alone. He used the Cosmic Cube to knock out the Mad Titan. It was embarrassing, to say the least.
2 Lockjaw — After the Pet Avengers used the Infinity Stones, Lockjaw sent Thanos to another dimension.
In an absolutely embarrassing loss, Thanos was defeated by a dog, albeit a super one. Lockjaw was a very cute dog, who formed a part of the Pet Avengers and his biggest win undoubtedly came against the Mad Titan.
At the time, the Pet Avengers had the Infinity Stones, and using them, Lockjaw sent Thanos to another dimension where he couldn't hurt a single soul. Maybe the Mad Titan took it easy seeing a group of animals.
1 Deadpool — Thanos lives to impress Lady Death, But she prefers Deadpool over Thanos.
Thanos' undying murderous rampage is to get in the good books of this superficial mythical Marvel being. But she prefers Deadpool, and Wade Wilson's presence itself keeps reminding the Mad Titan.
This is undoubtedly the worst defeat he has ever suffered. The whole point of him being this evil monster becomes pointless when there's someone else in the life of the girl he truly loves.