North Korea does have parties, some of which have seats in congress, but they’re small enough to be irrelevant.

Cuba has one party, but since every member of the party above the lowest level is elected, it functions as a more democratic apparatus than most multiparty systems, as evidenced by the overwhelming referendum they had on their constitution.

China is similar but more complicated.

Also while it does have only one party, there are factions within the party that are the functional equivalent of parties.