I just read a blog about one person's 5 most impact sermons in their life, so it got me thinking and I could think of 3.
1. First, I remember attending my first or second college meeting of the Iowa State Navigators in the Memorial Union. Ron Shimkus, a mid-50s, staff leader spoke from Revelation chapter 2. The passage included Jesus' direct words to the 1st century church in Ephesus. The entire sermon focused on Jesus' declaration that the church in Ephesus had "lost its first love." A "Christian church" that had quit loving Jesus is a church about to become defunct. It's true of churches and it's true of Christians. I was convicted that Jesus needed to be my first love. I needed to pursue Him in prayer, the Word, and obedience. The temptations of college grew dim in the light of the glory of Jesus.
2. The second "sermon" that stands out is actually a recorded lecture by Tim Keller that he first presented in 2003, but I listened to it sometime in 2008-2009. It's one of the few things I've ever listened to that I immediately relistened to, and then again, and then again. I've told people over the years that I've had 3 or 4 conversions in my Christian life. I believed in Christ Jesus for the first time around age 10-11, when I first heard of God's forgiveness through the Cross of Jesus and my need to personally repent and receive Jesus as Lord and Savior. I had "another conversion" when I was 15 or 16 when a preacher helped me totally rest in God's grace for salvation. I didn't need to "do good, Christian things" to be saved; I needed to find my only hope, peace, and joy in the Triune God--Father, Son, and Holy Spirit. Then in college, I was converted again through the various teachings of John Piper and college pastors and ministries in Ames, Iowa, that rightly taught that God is glorified most when I find my deepest satisfaction in Him. Worship was no longer duty, but a delight. My 4th conversion occurred, while jogging on the South Platte River trail in Littleton, CO, while listening to Pastor Tim Keller talk to me about the depths of the Gospel of Jesus Christ. You too can be "reconverted" by listening to this powerful lecture: http://www.gospelinlife.com/what-is-the-gospel-4621 (After you listen to this lecture, spend some time on the gospelinlife.com website, taking in all the free sermons available.)
3. The third "sermon" is once again, not a sermon, but a short message from Larry Austin to a leadership training class on a random Thursday evening Spring 2007 at the First Evangelical Free Church in Boone. I don't remember much, but I remember this line, "There is a huge difference between productivity and fruitfulness." That is, Christians and churches and pastors can do a lot of stuff, but it might end up having no eternal or spiritual value. All fruitfulness flows from intimacy with Jesus Christ (cf. John 15). If our souls become disconnected to Jesus, we offer nothing to those under our care. The teachings of Peter Scazzero (a fantastic 2-day event while I was at Denver Seminary plus his books), Dallas Willard (a week long class in seminary and a 2-day encounter in Green Lake, WI, plus his books), and Henri Nouwen (books alone) have helped me believe and practice this more and more.
I listen to 3-10 sermons per week while driving and jogging. Though I can't at the moment think of another sermon that stands out, I am indebted to the preaching ministries of Dan Leman, John Piper, Tim Keller, Mark Dever, and so many others (Dick Lucas, Alistair Begg, Paul Tripp, Zack Eswine, Don Carson, and the print sermons of Charles Spurgeon, Charles Simeon, Jonathan Edwards, Alexander Maclaren, John Calvin, John Wesley, and so many others.)