The default sort order for a forum can be adjusted within a forum's settings, though it's initially set to "Best".
Commenters have the choice to select their own preference for sort order by clicking the Discussion tab drop-down menu. To ensure your sort preference is remembered in the future, make sure that browser cookies are enabled.
Best - Comments with the most votes trending over time.
Newest - Most recent comments first.
Oldest - Oldest comments first.