what is the best hold item in pokemon platinum

[Chameleon]

what is the best held item in pokemon platinum?

[Zidarose]

The best hold item really depends on what you are doing.
For aggressive Pokemon, using a Choice Item (Choice Band, Choice Scarf, etc) helps, giving you a notable boost at the expense of being able to use one move.

For defensive Pokemon, Leftovers. This is probably the most common item I see in builds, giving anything holding it a way to recover a little life.

Aside from that, give Species Specific items to those Pokemon, look for things that boost what you are aiming to do and keep leftovers in mind for anything that has to soak hits.