Starlin wrote Thanos as Mary Sue character? Wut? There are a lot of instances where Starlin wrote Thanos as a humble character who was also humiliated both physically and mentally. And same goes for a lot of other writers. I mean, what the hell? The guy who had him wreck the Avengers (Hickman) also wrote him as a guy who proved to be inferior to his son Thane. That's Mary Sue? When he absorbed the Heart of the Universe and became Omnipotent in Marvel: The End, he couldn't save the Universe despite the fact that he was, well, Omnipotent. While he overpowered every single being in the Marvel Universe, it's pretty clear that Thanos was desperate in this instance, which is pretty ironic when you think about it considering the fact that he became the God. At the end of the story, Thanos actually sacrificed himself by recreating the Marvel Universe and curing it from so called "cancer" (heroes and villains keep coming back to life).
Similar thing happened in Thanos Quest, when Thanos assembled the Infinity Gauntlet, yet Death still refused to speak to him. At the end of that story we actually see that, despite the fact that he became the most powerful being in Marvel Universe, Death still did not accept Thanos. He actually said that he could make her do whatever he wants, but he wants her genuine love, and if he takes it by force, it's not the real thing. And at the end of the story we actually see Thanos shedding a tear, which is once again, pretty ironic considering the fact that he surpassed pretty much every being in the Marvel Universe. And this is the same issue where Thanos admitted that the Runner got the best of him based on both his dialogue and facial expression and agreed to provide the information he wanted (although Thanos defeated the Runner in this same issue, using the Time Gem at the end, so take that as you wish).
In the seventies, Thanos was defeated by Mar-Vell who smashed his Cosmic Cube and then was killed by Adam Warlock, who turned him into stone. During his first solo series, he was zapped by Galactus and Thanos admitted Galactus could have killed him if it wasn't for his energy sheild.
Those are not the only instances where Thanos was either defeated and/or humiliated in some way. He was overpowered by the likes of Odin, Tyrant and the Omega Thanosi in Warlock and the Infinity Watch Vol. 1 #25, Cosmic Powers #6 and Infinity Abyss #6 respectively (although the former two had a hard time putting him down which is impressive durability showing for Thanos). All those issues plus the ones already mentioned above were written by Jim Starlin and Ron Marz. See, there are a handful of instances when Starlin wrote Thanos as anything but "Mary Sue character". I mean, Starlin is the one who created Thanos, and he was created to be a powerful threat. In his first appearance, he wrecked Drax the Destroyer which destroyed the planet as a side effect (Iron-man Vol. 1 #55), and throughout most of his appearances he was introduced as a Teambuster, capable of wrecking Heralds like Surfer and teams like the Avengers composed of powerhouses like Hulk and Thor. And yet despite all that, Starlin had Thanos humiliated, defeated, overpowered and helpless more than once, even in an extremely ironic instances when he literally became God. Just because he wrecks popular and powerful characters, holds his own against Gods and has plan for pretty much everything does not make him a Mary Sue character. He is just a smart Teambuster. I mean, if he is Mary Sue, we might as well say that Ultron, Apocalypse, Mangog and other seemingly unbeatable villains who were stopped only thanks to deus ex machina are Mary Sue as well, especially because Thanos has more bad showings than these characters (not in terms of feats but in terms of introduction), as I've mentione above, all written by Starlin. And that's not even mentioning the other instances where he was depicted as nothing but a hopeless suitor to Death, showing signs of emotional weakness almost every time he is close to her and overall just looking pathetic. Or when his ultimate power was taken away from him more than once, like Cosmic Cube and the Infinity Gauntlet (although it was later confirmed by Vision and Warlock that he lost those powers, or at least the Infinity Gems only because he unconsciously wanted to, but still, he knew humility and defeat).
I just want to say that, if any writer made Thanos interesting and complex, it's Starlin. Under Starlin Thanos showed he actually has an interesting character, who is more than being powerful and evil, and actually has some human characteristics. Starlin's Thanos was actually respectful, reasonable and sometimes even heroic. He cares about Adam Warlock and Gamora, even if he's unable to be more nice about it. His intelligence may be called a plot device but same can be said for Doctor Doom and Batman.
Aside from Starlin, most of Marvel writers (with a few exceptions) think Thanos is this creepy guy who wrecks and kills everybody without a second thought, like he is a cosmic Ted Bundy. In other words, Marvel staff and casual fans thinks he is just Marvel's version of Darkseid or Mongul, while Starlin's Thanos is a completely different character with no similarities to Darkseid aside from appearance (Thanos wasn't even based off of Darkseid at first but another New God called Metron). While Thanos' most impressive feats can be found in Starlin's books, same can be said for instances where he has been humiliated.