What do you think?

Stating your opinion
I think...
I feel...
I believe...
I think it is fair to say...
In my opinion,....
I think it depends on....
Personally, I'd say that...
In my view,...
If you ask me,... 
For me,...

Agreeing
I think you're right.
I agree
Absolutely true.
I totally agree.
You are absolutely right.

Disagreeing
(I'm afraid) I disagree / I don't agree.
I'm not sure I agree with you.
I can't agree with you.
That's ridiculous/ absurd.