Most criminals in the United States are drug addicts.
Literal
America's criminals' most [topic-は] drug-addiction is.
ほとんど means 'most/almost all.' 麻薬中毒 is 'drug addiction' — 麻薬 (narcotics) + 中毒 (addiction/poisoning). である is the formal copula. A blunt factual claim in written register.