Titans Are Powerful
To understand what the Titans are, you need to understand the difference between an Eternal and a Deviant.
A race of god-like creatures called the Celestials had a habit of wandering around the cosmos, splitting races into their basic, Eternal, and Deviant forms. The reasons they did this have not been adequately explained in a way that hasn't been retconned to my knowledge, so let's just say they did it for fun.
The Titans are the "Eternal" form that humans take. Eternals are beautiful, powerful, smart. "Perfect." They are this controlled version of what perfection of a species looks like. As the Celestials spanned the universe, there are Eternal versions of other species, such as the Kree, as well as Deviant versions of other species, such as the Skrulls (assuming they show up in the MCU at some point).
But Thanos is No Mere Titan
Thanos is a Titan, and therefore of a breed that is perfect. But the Celestials also left Deviants. Whereas Eternals are perfect, Deviants are chaos. Their genes mutate from generation to generation, creating wildly different looks between members of their race (Deviants are often hideous) and the possibility of super powers.
Thanos simultaneously has these perfect Eternal genes, giving him great power, but also carries Deviant genes, which also gives him great power. This is also why he is purple, and looks nothing like either of his parents (the chaos of a Deviant gene).
Nobody compares to Thanos.