In my little world, Steve Earle's work can be divided into three periods.
For the best of his early, pre-addiction period , check out "Copperhead Road".
Then for the best of his clean-and-outta-jail work, you have to have his three best albums which are:
"El Corazon" -- worth the money just for his duet with Siobhan Maher-Kennedy (ex-River City People) on "Poison Lovers"
"I Feel Alright" -- complete with a duet with Lucinda Williams on "You're Still Standing There ")
"Transcendental Blues"
From the same period, I'd also recommend looking out for his duet with the marvellous Iris DeMent ("I'm Still In Love With You") which is on a album he recorded with the Del McCoury Band.
The last and current period is his too self-righteous-for-words period. I'm not too enamoured of this really. But the recommended release has to be "The Revolution Starts Here".