Just so. Quite so. I quite agree here. Naturally. Certainly. Sure. Exactly. Definitely. Quite. Most likely. Absolutely. True enough. I couldn't agree more.
I should think so. Beyond all doubt. I won't deny it. Looks like that. Happy to hear it.
I'm not sure I quite agree. Why do you think that...? I'm afraid I don't agree.
I think you're mistaken (there). I don't think you are right. I can't agree with you there.
I see what you mean, but... I'm not so sure. I see nothing exciting in ...
I doubt it. I (you) shouldn't say so. There's something in what you say, but
I disagree with you. You're wrong. You're mistaken. Not me!
How can you say such a thing! On the contrary! You can't be serious.
I object to it. Surely not. Nothing of the kind (sort).
Just the other way round. Certainly not. Impossible.
It's unfair. It's unjust.
Giving Advice
Might it be an idea to ...? Have you ever thought of You could always ... If I were you, I'd ... Why don't you ...? You'd better ...