John Calvin, Commentary on the Book of Isaiah -- Solid and helpful mediation on the text.
John Gill, The Book of the Prophet Isaiah -- Interesting and edifying.
Martin Luther, Lectures on Isaiah -- Homiletically and historically helpful.
J. Alex Motyer, The Prophecy of Isaiah -- While not a bad commentary, I was surprised to find myself not using it much. I suspect Motyer has over analyzed the structure and relies too much on his assessment of structure for the meaning rather than the text.
John N. Oswalt, The Book of Isaiah, NICOT -- Became the backbone of my sermon series. Oswalt is Arminian, but incredibly helpful. Please also see my review of his The Bible among the Myths: Unique or Just Different?