You could decide the best book of seera when you decide the purpose of your reading.

Outlines of Seera

If you are a non-specialist who just need to have general thoughts about the lifetime of the Prophet (saw) and the way he preached, you could read:

• Ibn Kathir, al-Fusul fi Seerat al-Rasul;
• Mubarakpuri, ar-Raheeq Al-Makhtum.

Western Reader

Non-specialist non-Muslim Western readers mostly need to analysis behind the events more than just its knowledge as well as the western misconception about the Prophet (saw) and his life. The books of Karen Armstrong are widely recommended even, like any writer, if she has some downsides:

• Karen, Muhammad: A Biography of the Prophet (most important);
• Karen, Muhammad: A Prophet for Our Time.

Scholarly Details of Seera

Here the issue became more complicated and enthusiastic. If you are targeting scholarly details and you want to read only one book, it would be 'Ibn Kathir, as-Seera al-Nabawiyah.' I always review it when I want to save time and only need to see most of the issue surrounding a certain event, especially when I read the issue in detail before and I need only to recall.

The features of this book are: The author mostly includes Hassan narrations; he does not exclude the narrations that disagree with his thesis, rather he includes and discuss; the book is more comprehensive as it includes almost about 80 % of the information you need to know about a certain event; unlike most of the contemporary books, it is not biased to certain trends.

But if you want to leave no piece of information regarding a certain event, you seriously need to review the following books, besides the above (in this order):

• Ibn Hisham, as-Seera al-Nabawiyah;
• Ibn Ishaq, as-Seera wa al-Maghazi;
• Tabari, Tarikh at-Tabari;
• Ya'muri, 'Uyun al-Athar;
• Kinani, al-Mukhtasar al-Kabir;
• Ibn al-Athir, al-Kamil fi at-Tarikh;
• Sohili, al-Rawd al-Aneef;
• Ibn Hiban, as-Seera al-Nabawiyah wa Akhbar al-Khulafa';
• Bayhaqi, Dala'il al-Nubuwah;
• Dhahabi, Tarikh al-Islam;
• Darami, as-Seera al-Nabawiyah

When you read these books regarding certain even, you would be able to analyze, check the authenticity, compare narrations and know the lost part in each narration and you will finally have a bird vision on the events of seera.

Conclusion

I can't say here is the best source for everyone to read; it depends on your purpose of reading. There are books are more likely suitable to scholarly details; books for general leaders; books for Western readers.