In recent history my personal fave has been Romans. I love the letter to the church in Rome mainly because I have come to love the way that the Apostle Paul explains what He has learned about the sovereignty of God over all of His creation. I rejoice the most in the knowledge that I now stand forgiven and innocent before Holy and Almighty God because of what Jesus did for me. Since God did for me what I could not do for myself, no one can undo what God has done and is doing in my life. I love how rock solid those promises are for us in Christ.
Ephesians is one of Paul's last letters (if not the last). We should be able to expect to see in Paul's understanding a noticeable growth as his understanding is shaped by fellowship with Christ in the Spirit. It seems to me that much of the beginnings of his letter to the church in Ephesus is spent trying to write words that would describe the indescribable desires and revelations that have been given to Paul.
It has given me a fresh perspective on how vast and immeasurable is the knowledge of God. God is ... like ... huge. There are so many wonderful words we could use to describe Him and all of them only catch a glimmer of His radiance. With this fresh perspective of God's holiness has come a fresh embrace of the news of the life, death, burial, and resurrection of Jesus Christ. It is by faith in Christ that we have been brought back to God. Now we can begin to see Him for who He is. How exciting!
In Paul's letter to Ephesus He says a prayer for the saints. In this prayer we get a glimpse of how immense and profitable our union with Christ is for us.
I've been chewing on this for a while and it never loses it's flavor. We are His body, the fullness of him who fills all in all. How can we possibly live in a manner worthy of such a calling?
15 For this reason, because I have heard of your faith in the Lord Jesus and your love  toward all the saints, 16 I do not cease to give thanks for you, remembering you in my prayers, 17 that the God of our Lord Jesus Christ, the Father of glory, may give you a spirit of wisdom and of revelation in the knowledge of him, 18 having the eyes of your hearts enlightened, that you may know what is the hope to which he has called you, what are the riches of his glorious inheritance in the saints, 19 and what is the immeasurable greatness of his power toward us who believe, according to the working of his great might 20 that he worked in Christ when he raised him from the dead and seated him at his right hand in the heavenly places, 21 far above all rule and authority and power and dominion, and above every name that is named, not only in this age but also in the one to come. 22 And he put all things under his feet and gave him as head over all things to the church, 23 which is his body, the fullness of him who fills all in all.