There is no such thing as a bad magic item - provided, of course, that it's not cursed or otherwise out to kill you.
Why's that, you ask?
Because even the holiest of holy avengers* can be sold for a pretty penny to a paladin in someone else's party, leaving you with great gobs of coin to spend on other things. Before this, the avenger* is merely a very efficient means of carrying around a lot of wealth without encumbering yourself.
That said, one of my least favourite items are those annoying Ioun Stones, mostly because having something like that flashily orbiting around your head also paints a big neon "Wealthy! Rich! Mug and rob now!" target on the owner of said head....
* - feel free to substitute here any magic item you otherwise dislike and would never use.