While Kang the Conqueror is quite formidable as a time-traveling despot, even he's afraid of Thanos the Mad Titan in Marvel Comics, making a point to avoid him at all costs. While this is a pretty good rule for most beings in the Marvel Universe, it's interesting that even someone as powerful as Kang is scared of Thanos. However, while Thanos: The Infinity Siblings confirmed Kang's fear of the Mad Titan, it also saw Kang and Thanos meeting face to face for the first time when the time-traveler decided to risk a meeting with the being who once wiped out half of all life in the universe.
In Thanos: The Infinity Siblings from Jim Starlin and Alan Davis, the majority of the story is focused on the relationship between Thanos and his brother Eros (AKA Starfox), while also offering a look at what Thanos could evolve into in the future, replacing the embodiment of Death itself and gaining immense power as witnessed by Kang the Conqueror. However, even before Kang saw Thanos' future self, he revealed to Eros that he's never had an interest in meeting his more volatile brother: "I have always made a strict point of avoiding Thanos of Titan. Continuing to do so seems prudent."
However, Kang eventually decided to chance a meeting with Thanos, despite his policy, near the end of The Infinity Siblings storyline, though it was still a choice founded by his fear of the Mad Titan. Meeting with Thanos, Kang proposed a deal where he would act as an advisor, offering Thanos insights into the future that would allow him to lay claim to the entire universe, provided Kang was allowed to rule his own empire in the distant 42nd century unopposed. However, before their deal was even struck, it was revealed that Thanos was already several steps ahead of Kang, expecting the time-traveler to come before him and anticipating his proposal.
What's notable about Kang's fear is that while Thanos does have great power and is incredibly cunning, it's not as if Kang doesn't have power himself. Being a time-traveler, he arguably has more power considering the endless opportunities and resources he has at his disposal. By traveling into the past, Kang the Conqueror could rewrite reality in the present. Likewise, by traveling into the future, he could learn what comes next and act accordingly back in his present to his ultimate advantage. These options and more could easily be used as offensive strategies against the Mad Titan, whose time-traveling abilities pale in comparison.
However, Kang's caution does seem to be well-founded. He's apparently seen enough to know that he could still be bested by Thanos regardless of his time-traveling capabilities. Thanos is a cosmic being who's wielded the Infinity Stones after all, and Kang has also witnessed what he could evolve into as well. As such, Kang's fear of the future Thanos is likely a founded one as he nows better than anyone what Thanos is potentially capable of in the future.