If you strongly disagree with someone's opinion, you can say this:

I couldn't disagree more.

Of course, disagreeing with someone so clearly may start an argument so you should be careful when you use this phrase. Use it when someone has said an opinion that you think is dangerous and completely wrong.

You can also say the opposite:

A: I think that this is a great opportunity for both of our companies.

B: I couldn't agree more.

"I couldn't agree more" sounds very positive.