Choice scarf is the absolute best option for a dittos held item in competitive play, as without it, it would be a speed tie, giving the original Pokemon a chance to attack. Quick claw is another option if you really don't like choice scarf, but choice scarf is the best. If you are using trick room, then your best option is the macho brace if possible, or a power item like the power lens as it has the opposite effect of lowering your speed, and unlike room service and iron ball, does not become useless upon switching nor does it ground you if you transform into a flying or levitating Pokemon. However, Macho Brace is only available in natdex formats, gen 8 OU, gens 6-3 OU, and more notably, their doubles counterparts when applicable. However, in the unlikely event you're using this to breed, know that Imposter Ditto cannot pass down hidden abilities.
Source: The comments, experience, and this.
