The single biggest turning point in the series
As far as how the drug war on the streets goes. And as I think about it it has to be Mcnulty deciding to visit Deangelo's girl and telling her that his death couldn't have been a suicide. Because following from that single event is her telling Brianna, Brianna putting pressure on String and Avon about it, and then String finally deciding to tell Avon about having him killed. If these things had not occurred, would String still have dimed on Avon, and Avon given String up to Mouzone and Omar? Maybe the latter but imo not the former. If either occurred.
And therefore if those two stayed together imo the Barksdales definitely win the war with Marlo and who knows how things progress from there. A whole slew of changes occur without Marlo being the King in S4 and 5.