The Tsar Bomba was an experimental weapon, but the United States had the B41, a three-stage thermonuclear weapon with a yield of 25 megatons. Four of these would have been as powerful as the Tsar Bomba; the U.S. manufactured 500 of them. http://en.wikipedia.org/wiki/B41_nuclear_bomb
In the 1960s the U.S. Strategic Air Command ran a continuous airborne nuclear-armed patrol (Operation Chrome Dome), so there was probably a 25 megaton weapon in the air for much of that time. http://en.wikipedia.org/wiki/Operation_Chrome_Dome
It's also worth remembering that the limiting factor on the yield of the Tsar Bomba was the carrying capability of the Tu-95. The Teller–Ulam design for thermonuclear weapons is scalable: you can keep adding stages as long as you have the uranium, plutonium and lithium to make them with. Edward Teller supposedly designed (or at least contemplated) a 10 gigaton bomb: http://blog.nuclearsecrecy.com/2012/09/12/in-search-of-a-big...
In the 1960s the U.S. Strategic Air Command ran a continuous airborne nuclear-armed patrol (Operation Chrome Dome), so there was probably a 25 megaton weapon in the air for much of that time. http://en.wikipedia.org/wiki/Operation_Chrome_Dome
It's also worth remembering that the limiting factor on the yield of the Tsar Bomba was the carrying capability of the Tu-95. The Teller–Ulam design for thermonuclear weapons is scalable: you can keep adding stages as long as you have the uranium, plutonium and lithium to make them with. Edward Teller supposedly designed (or at least contemplated) a 10 gigaton bomb: http://blog.nuclearsecrecy.com/2012/09/12/in-search-of-a-big...