This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
That depends greatly on what you're doing with it. Arrays offer slightly better performance (there is less overhead to deal with), but ArrayList is more flexible (you can add and delete elements). I prefer Collections for their convenience, which to me is almost always more important than speed.