Kinda surprised no one has at least mentioned time stop. Possibly not the strictly worst spell, especially since it's defensive and flavorful in nature, but it does actively prevent you from dealing damage. One could say, it has a "negative damage per round" cost.
It costs you 1 Action to gain 2-5 actions in which you cannot affect someone else. It might be a great buffer spell, except that most personal buffs require concentration. And you can do nothing to help your allies, either. There are several situational uses, of course, plus the coolness factor, but a relatively useless spell, considering what you can accomplish with 1 Action.
[MENTION=6799753]lowkey13[/MENTION], I'd like to add to the list!