In Guardians of the Galaxy, when Ronan got stone of infinity on Knowhere and decided to not give the stone to Thanos, Korath told him that Thanos is the most powerful being in the universe. Why is he so powerful? His only abilities so far I can see are achieved using the stones in Avengers. His only visible power is strength, but without stones he would not have chance even against most Avengers in 1v1 fight.
I didn't read any Marvel comics, I've just seen the films, so are there any abilities he doesn't use in the films? Or why is he considered the most powerful being if he seems so weak without the stones (compared to Avengers, for example)?