4, 1, 3, 5, 2
Upon finishing The Wire during its initial run on HBO, that's the order I thought I'd rate the show's seasons from best to... well, I can't say "worst," as the nadir of The Wire was still a cut above anything else on the tube.
However, after watching the complete series a second time, the seasons are all jumbled together, and the sections I initially felt any disappointment about, I found deeper value upon a repeat viewing. The fifth season, for example, I initially docked for too much on-the-nose dialogue in the newsroom scenes, but from a macro standpoint the season is genius: everyone gives in to the irresistible temptation to "juke the stats" a.k.a. lie: McNulty and Freamon in the homicide department, Templeton, Clavanaugh, and Whiting at the newspaper, Pearlman and Levy in the courtroom, Burrell and Rawls in the police administration, and Carcetti and Steintorf in the mayor's office. In an era of baseball players injecting steroids and a president lying to get us embroiled in a war, it's easy to see how the individual urge to get ahead makes one calculate that the ends justify the means.
If I now find myself having a hard time evaluating the seasons separately, it's because "all the pieces matter" and the seasons fit together closely as do chapters in a book. Indeed, while I could write a million words about The Wire, I'll just cut this short by saying that the show changed my outlook on Television as a storytelling medium, and the DVD box set would feel more at home on a bookshelf next to the literary classics than it would next to other TV shows.