Way 1 · best default
I'm not sure I agree
A soft, honest no. "Not sure" leaves room for the other person to explain.
Examples
I'm not sure I agree. I think the first hotel was better value.
Hmm, I'm not sure I agree with that, to be honest.
In a conversation
A: Working from home is just less productive.
B: I'm not sure I agree. I get loads more done at home.
Common mistake
Not: I'm not agree.
Say: I'm not sure I agree.
"Agree" is a verb, so it's "I don't agree", never "I'm not agree". The "not sure" version is also softer.