Casual:
* Nope
* Nah
* Negative
* Not a chance
* No way
* Absolutely not
* Forget it
* Not happening
* Not on your life
* Not in a million years
Formal:
* I decline
* I refuse
* I disagree
* That's not possible
* That's out of the question
* I cannot comply
* I am unable to
More Creative:
* Not even a little bit
* Not for all the tea in China
* Not in this lifetime
* Over my dead body
* No way, José
The best way to say "no" depends on the situation and the relationship you have with the person you are talking to.