1. I don't think so.
2. I beg to differ.
3. I'm afraid I don't agree.
4. I'm not so sure about that.
5. That's not how I see it.
6. Not necessarily.
7. Yes, but... [say your opinion]
8. On the contrary.
9. (very informal) No way!
10. (very strong) I totally disagree.

Fact
In 1900, Queen Victoria sent her New Year's greetings to the British troops stationed in South Africa during the Boer War in the form of a specially molded chocolate bar.      .. More >>