The beta stops at level 30, so "best in slot" right now means the best gear you can get at 30. The lists below come from Wowhead's beta Beast Mastery guide, which was written from hands-on play. We list item names only; check stats in-game, because Blizzard is still retuning quest rewards (the 26 Oct notes corrected the item level of several). Wowhead lists these for Beast Mastery. Note that the Greater Friend of the Library chain also gives Truthseeker's Bow, which the 8 Oct notes moved to a level 40 requirement, so it isn't usable at 30. We haven't found sourced lists for Marksmanship, Survival or melee hunter. Those lists: Not yet published, updated at launch.
Leatherworking