The largest US test was Castle Bravo at 15Mt and that was far more than the expected yield of ~6Mt (which caused a lot of problems):

http://en.wikipedia.org/wiki/Castle_Bravo

The Soviets did test a 50Mt version of a 100Mt design - the "Tsar Bomb":

http://en.wikipedia.org/wiki/Tsar_Bomba

The US did have a 25Mt weapon which was a real "3 stage" design (i.e. primary/secondary/tertiary - presumably both the secondary and tertiary being fusion/fission).

http://en.wikipedia.org/wiki/B41_nuclear_bomb

The USSR had SS-18s with very large warheads (20-25Mt) - presumably to hit deep bunkers (Cheyenne Mountain, Raven Rock Mountain etc.).