It really depends on what type of witchdoctor they are playing, as a staffy witch will be using different gear than a staffy shooty witch or a staffy melee witch. Different players also prefer different abilities from each other, and some simply go for stats. The point is, there isn't any one best gear of any item for a witchdoctor as it all boils down to how a player wants to build theirs. Granted, most will want to farm Captain Blood's Jacket due to Blood Flames being OP (a whole lot of players would like to see Blood Flames not be OP despite it being banned in PvP... hint: if it has to be banned in PvP then that is a clue that it likely needs to be looked at). If you go staffy, then the scorpion staff farmed from Moo Manchu is a viable choice, though many go with Phule's wand.

What I see occurring is that people who play Wizard101 are so used to everyone going after the same equipment cause there tends to be equipment and equipment sets that are best for all wizards of a school, but in Pirate101 and the appearance of the secret trainers, people have tossed aside what equipment is best for a Pirate class so that they could customize their pirates into hybrids which tend to enjoy more.

Realistically, they could use nothing but Bazaar gear.
However, when it comes to "best" gear two things come to mind: Captain Blood's Jacket (and the Obsidian alternative) and Phule's Wand (or Moo Manchu's Wand).

Both of these items give immense power to a Witchdoctor. The first gives Blood flames which is just a super strong ability. The second gives 6 base range plus a free Witchhunter rank (the Moo Manchu wand gives cowards bane).

Besides these, any items with Juju, heals, protections, or other will-based buffs are also great items for a Witchdoctor.