Agreeing

"I think you're right."
"I agree with you."


Strong agreement

"I couldn't agree with you more."
"You're absolutely right."
"I agree entirely."
"I totally agree."


Agreeing in part

"I agree with you up to a point, but..."
"That's quite true, but..."
"I agree with you in principle, but..."


Disagreeing

"I'm not sure I agree with you."
"(I'm afraid) I don't agree."
"(I'm afraid) I disagree."
"(I'm afraid) I can't agree with you."
"(I'm afraid) I don't share your opinion."

Note that when you disagree with someone, you can often sound
more polite by using a phrase such as "I'm afraid..."

Disagreeing strongly

"I don't agree at all."
"I totally disagree."
"I couldn't agree with you less."