To express an opinion :
I think…
In my point of view / my opinion,….
As I see it, …
According to me,…
To my mind,…
It seems to me that…

Agreeing with an opinion :
I agree with this opinion.
This is absolutely right.
I completely agree with this view.

Disagreeing with an opinion :
I disagree with you.
I don’t agree with you.
I think you’re wrong.