This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
I don't think it is possible to choose just one. I personally used quite a few. If you somehow managed to get me in a choke hold and demanded that I answer the question... then I would probably pass out. On the other hand, if you managed to get me in a Figure Four Leg Lock and demanded that I answer the question... then I would probably say Mastering EJB or Enterprise JavaBeans. However, no wrestling move could force me to choose between the two.