The largest bomb ever lit by the US (at Bikini) was 15-16 megatons which was about twice as big as they expected due to the unexpected interaction of the lithium in the fusion process. That must have been a real pucker-factor increaser, can you even imagine?
However, the USSR tested one with a design yield of 100-150 megatons, damped down to 50 for the test in a rare display of common sense for the Soviets.