One Decade, One Song - Petra

70's - Lucas McGrawn
80's - Grave Robber 
90's - Creed
00's - Jekyll & Hyde

70s: Walkin’ in the Light
80s: You Are My Rock
90s: Beyond Belief
00s: Woulda, Shoulda, Coulda

80s Judas Kiss
90s Creed/ Armed and Dangerous
00s Jekyll and Hyde

70's: Come and Join Us.
80's: Beat the System.
90's: Beyond Belief.
00's: Test of Time.

70's: I’m Thankful
Early 80's: It Is Finished
Late 80’s: You Are My Rock
90's: Creed
00's: Jekyll and Hyde

This is hard where they released so many great songs/albums:

70s - Magic Words (killer guitar solo)
80s based on singer - Greg Volz Second Wind, John Schlitt Hit You Where You Live
90s - I could pick Beyond Belief or Creed but I'll go with Destiny - killer opening and sustaining riff throughout the song.
00s - Jesus Friend of Sinners is a good song that Petra didn't write but I'll got with Jekyll and Hyde
10s - Too Big to Fail