Russia
Russia tested its first nuclear weapon in 1949's “RDS-1,” as it attempted to balance out the West's advantage during the Cold War. It tested its first hydrogen bomb in 1957's “RDS-37.” It also had the most powerful bomb ever exploded in Tsar Bomba.
Before the Soviet Union collapsed, its arsenal included 45,000 nuclear warheads.
Russia has trimmed it down to 8,500, though only 1,740 are active.