These are my top five, in no particular order:
1. The Dhammapada
My favorite is the Thomas Byrom verse rendering published by Shambhala. It's beautifully poetic and accessible, and also conveniently pocket-sized - my copy lives in my purse!
2. The Heart of Understanding by Thich Nhat Hanh
Though I've never had the honor of meeting him, I consider Thich Nhat Hanh to be my first Dharma teacher. He has a wonderful gift of making difficult concepts easy to understand without any "dumbing down," and you don't have to be a Buddhist practitioner to benefit from his teachings.
3. The Lotus Sutra
I first read (and reread and reread) the Burton Watson translation; I'm now reading the new translation by Gene Reeves. It's one sutra I continually return to; the sense of wonder I find in it never goes away.
4. In Praise of Tara: Songs to the Saviouress by Martin Willson
This is the best collection of writings, prayers and praises devoted to the Buddha Tara, my patron deity.
5. The Tibetan Book of Living and Dying by Sogyal Rinpoche
Sogyal Rinpoche addresses a Western audience in this book about how we in the modern world relate to life, death, and dying. It's helped me through a lot in recent years.
That's my list - I would love to hear what Dharma books are most important to you all!