I don't agree with you./I don't agree at all.
You're completely wrong.
You couldn't be more wrong.
You've got it totally wrong.
I'm afraid you missed the point here.
I won't go along with this.
It would never work.
I doubt that's true./I don't think so./I don't think that's true.
That can't be right.
I don't see it that way.
We don't see eye to eye on this issue.
A: What do you think of this plan?
B: Have you thought of the consequences of your idea?
A: Okay, that might work, but don't you think there's a better solution?
B: I think John has a better solution to this problem.
A: That's my suggestion to this issue.
B: I understand where you're coming from, but I think there's a better solution.