In preferences, you probably want "Mixed Order."

Sort Orders:
Oldest First Gives you a list in which threads (or messages, depending on your listing preference) are arranged with the oldest at the top of the page and the newest at the bottom.
Newest First Does the opposite. The other two options are relevant only to threaded and compressed listings.
Mixed Order Arranges primary messages with the newest at the top, thus tending to keep newer threads toward the top of the page, but arranges responses with the newest at the bottom, thus preserving a more "intuitive" threading structure.
UBB Order Arranges threads by the dates of their last responses, rather than by the dates on which they were started; this ensures that the threads with the newest messages are always at the very top of the page, but also means that threads are constantly "reordering" themselves on the index page.