Formal/Legal:
* Disagree: This is the most general and straightforward opposite of "concur." It means to have a different opinion or to not agree.
* Dissent: This is a more formal word used to describe a disagreement, especially in a legal or political context. It often implies a strong disagreement with a majority opinion.
* Oppose: This implies actively taking a stand against something.
* Refute: This suggests a strong and logical disagreement that aims to disprove an argument.
Informal:
* Disapprove: This implies a negative opinion about something.
* Disagree: (as above)
* Object: This means to express a formal objection to something.
* Differ: This is a milder way of saying "disagree." It simply indicates a difference of opinion.
Other:
* Contradict: This suggests that something is in direct opposition to another statement or idea.
* Dispute: This implies a disagreement that is being argued over.
The best word to use will depend on the specific situation and the level of formality required.