I think I've found the absolute worst promotional product ever: the antimicrobial light switch cover. Because everyone wants a company's logo on their light switch cover. And we're all tossing and turning over fears of germy light covers. Who would ever want this thing?
As to the best promotional products, well, that depends on the event, the audience, etc., etc. I generally prefer to give products that are useful (and hopefully won't go right into the trash) and will be seen by multiple people. If they tie into the theme even better.
It would be nice to always be able to give something new and exciting, but those items usually aren't so useful. Umbrellas were probably the most complimented item I've given out. When I'm on the receiving end I tend to keep the useful items like portfolios, bags, travel mugs and pens while the more flashy, but useless trinkets are purged.
What promotional products have you found to be most useful?