I think there is no "best" giveaway... it all depends.
It depends on who your target audience is... are they senior citizens or moms with toddlers? Are they skate boarders or librarians?
And what can you afford to spend per giveaway? Do you need a real cheap giveaway or can you spend $20 on each one?
I think the key to picking good trade show giveaways is making sure your company name and website are printed on them, and that the giveaways are useful and that people will keep them instead of tossing them in the trash when they get back to work.
Bottom line... as with life, there really is no simple answer, but focus on picking a trade show giveaway that people will use and keep.