Kang's True Endgame Proves He (Not Thanos) Is the Avengers' Real Nemesis
While Thanos is by far the most famous of the Avengers' villains in 2022, the main nemesis of the team has always been Kang the Conqueror - and there is indeed no contest regarding who is the better villain. Thanos is the more famous of the two, especially with the blockbuster success of both Avengers: Infinity War and Avengers: Endgame in 2018 and 2019 respectively. But Kang is absolutely the most dangerous villain toward Earth's Mightiest Heroes - simply because he desires nothing less than the defeat and death of the Avengers.
Both Thanos and Kang and incredibly dangerous threats to the team and to the Marvel Universe at large. Thanos commits evil actions for the purposes of courting Death (who in the Marvel Universe is an actual entity and the personification of the cessation of all living things); he gathers the Infinity Stones in 1991's Infinity Gauntlet mainly to impress her. Killing half of all life in the universe is simply one more way to win her favor (in the MCU variant, this is his only motivation, as he sees the universe as overpopulated). While Thanos concerns himself with Death and other cosmic forces, he regards the Avengers as little more than annoyances. The same cannot be said for Kang.
While Thanos sees the Avengers as a constant obstacle to his goal, Kang sees the Avengers' destruction as the very meaning for his existence. Almost all of his plans revolve around the team's defeat. The Avengers #71 reveals that Kang's desire for revenge against the team trumps even his love for Ravonna. The Grandmaster gives Kang a choice: to be granted the power over Life or Death, and so blinded is the conqueror with hate, he chooses the power of Death, foregoing his chance to revive Ravonna. Kang is not an inhuman monster - he has compassion and love for another - and he still chooses to kill the Avengers over bringing back a loved one.
This naturally raises the question: does Kang's laser focus on the Avengers make him a better villain than Thanos, who doesn't even regard the team as a major threat? While there is something to be said about a villain who is not motivated by a personal vendetta (unlike so many other comic book antagonists), Kang is a much more dangerous foe. If the events of Avengers: Mech Strike are any indication, Kang has the power to kill Thanos and not even consider the victory worth mentioning on the road to defeating the Avengers for good.
While Thanos requires the Time Stone to manipulate time, Kang can very famously travel through time at will. He is a brilliant scientist when it comes to time manipulation and indeed can even built a fortress outside of time, as seen in Timeless #1. This ability, in addition to his pure hatred for the Avengers, makes Kang a much more formidable foe than Thanos, and the single biggest individual threat in the Marvel Universe.