I agree: Example: I agree with you; that's a valid point.
I concur:
Example: I concur with your assessment of the situation.
You're right:
Example: You're right; that makes a lot of sense.
I agree with you wholeheartedly:
Example: I agree with you wholeheartedly on that matter.
I couldn't agree more:
Example: I couldn't agree more with what you just said.
That's a good point:
Example: That's a good point, and I agree with it.
I share the same opinion:
Example: I share the same opinion as you regarding this issue.
I'm on the same page:
Example: We're on the same page; I think that's the best approach.
You've got a point:
Example: I hadn't thought about it that way, but you've got a point.
Absolutely:
Example: Absolutely, I agree with your perspective.
Certainly:
Example: Certainly, your viewpoint is valid.
Indeed:
Example: Indeed, I think you're absolutely right.
You've hit the nail on the head:
Example: You've hit the nail on the head; that's exactly how I see it.
You're spot on:
Example: You're spot on with your analysis of the situation.
I'm in complete agreement:
Example: I'm in complete agreement with what you're saying.
You're absolutely right about that:
Example: You're absolutely right about that, and I fully support your viewpoint.
I'm of the same mind:
Example: I'm of the same mind; I think we're on the right track.
Couldn't have said it better myself:
Example: Couldn't have said it better myself; you've captured the essence of the issue perfectly.
I'm on board with that:
Example: I'm on board with that idea; let's move forward with it.
I'm with you 100%:
Example: I'm with you 100%; your perspective aligns perfectly with mine.
You've convinced me:
Example: You've convinced me; I see things differently now.
You're onto something:
Example: You're onto something; that approach could work well.
You have my full support:
Example: You have my full support on this matter.
I'm inclined to agree:
Example: I'm inclined to agree with your assessment.
You make a compelling argument:
Example: You make a compelling argument, and I'm persuaded by it.