In hindsight season 2 was the most cohesive season out of all of them. Introduced and expanded on great characters like Frank Sabotka, Valchek, Got to see Bunk and Lester team up, McNulty's self-destructiveness on full display. Everything tied up at the end, all of it started because of Valchek's buffoonery. For me it's gonna be 1 - 2 - 4 - 3 - - - 5.