Re: worlds most powerful NUKE SUB
The USS Nebraska is one of 14 Ohio class SSBN's in the USN. Four of the class have already been (or are currently undergoing) refitting to guided missile subs with the added ability to deploy special ops forces. The fleet has been reduced per agreements with Russia on nuclear weapons reductions. It was decided to convert the four subs rather than put them in mothballs, since they still had a lot of life left in them.
Ohio class is indeed a powerful submarine, each boat carries 24 Trident II nuclear missiles. Each missile can carry 8 independently target warheads, and has a range of 4,000nm. About 50% of the US nuclear deterrent rests in the Ohio class.
The Russian sub fleet has suffered almost total decay due to lack of funding, the boats that are considered theoretically seaworthy are generally unable to deploy due to lack of crews and maintenance, and aging weapons.
China's has only one XIA class SSBN, which is considered obsolete. IIRC, Their new Type 094 is reported to be launched, but does not have any missiles, so it sits in port. China's nuclear submarine fleet has suffered from reactor problems and missile problems, and is not considered a credible force.
Given the decrepit state of the Russian fleet, and the lack of missiles and platforms in the PLAN, there is no other country that comes even remotely close to the US in Fleet Ballistic Missile Submarines.