I plan to set a side quest into the adventure of the starter set. It is started by a riddle the party has to solve in the beginning of the campaign. The quest reward should be a magical item that feels really awesome, while not decreasing the difficulty of the adventure too much. So drastically increasing stats or granting additional strong powers is not what I'm looking for.
So: Are there any items that feel notably powerful while not contradicting the restrictions mentioned before? If so, are any of them useful for all characters, regardless of race/class?
I'm not expecting you to make a list, but rather a reference to such items in official material, so that I can look up the best fit for my self.