The Bible

Ok, it is not exactly a devotional, but we at BestDevotionals.com always like to start with this one at the top of our list.
Open to anywhere and instead of just reading, look for who God is in the situation described etc. Get to know Him better as you read.
Let the Holy Spirit inspire you--it is not always as simple to do this as you might think. Here is a version we recommend for readability
and accuracy of things like various numbers (due to having older manuscripts available)--this is also less expensive than most devotionals!
My Utmost For His Highest

The classic that never gets old. Oswald Chambers shares thoughts on a verse each day. One of the most popular devotionals ever.
(Insider note: this work was actually a team effort. After Oswald Chambers' death, his wife compliled his writings and her notes from his lectures and put together this devotional).
The Purpose Driven Life

We highly recommend this popular book by Rick Warren, where we learn it is not about us.
For His Glory
How can we cultivate a mindset of bringing glory to God?
Find out in this 30 day devotional by Colin Chalmers. Or read it straight through as a book--either way it is sure to encourage your relationship with the Lord.
God's Wisdom for Navigating Life

Tim and Kathy Keller have put together a one year devotional based on the book of Proverbs!