Agreeing. Neutral: Yes, I agree. True enough. That's right I can't help thinking the same. Hour true. I couldn't agree more. How right that is. Oh, definitely.
Informal: Well, that's the thing. Well, this is it (isn't it) ? Yes, right. Dead right. Too true. I'd go along with you there. I'm with you there.
Formal: Oh, I agree entirely. I agree absolutely with... My own view/opinion exactly. I'm of exactly the same opinion. I don't think anyone could/would disagree with...
Disagreeing. Neutral: (Oh,) I don't agree... I'm not (at all) sure, actually/in fact. Not really. Oh, I don't know. No, I don't think... I disagree (I'm afraid). That's not right, surely. That's not the way I see it. I can't agree with... I can't help thinking... But isn't it more a matter/question of... ? Do you really think...?
Informal: (Oh) surely not I don't see why. I can't go along with... (Oh,) come off it. Nonsense! Rubbish! No way! You must be joking. You can't mean that!
Formal: I really must take issue with you (there). (I'm afraid) I can't accept... I can't say that I share that/your view. I'm not at all convinced... I see things rather differently myself.
Saying you partly agree. Neutral: I don't entirely agree with... I see your point, but... I see what you mean, but.. To a certain extent, yes, but... There's a lot in what you say, but... Yes, maybe/perhaps, but.. I couldn't agree more, but... That's one way of looking at it, but... Yes, but on the other hand, ... Yes, but we shouldn't forget... Yes, but don't you think... That's all very well, but...
Informal: Could be, but... OK, but... Yes, but... Mm, but... I'd go along with most of that, but...
Formal: Well, while I agree with you on the whole, ... There's some/a lot of truth in what you say. Still/however, ... I agree in principle, but... That may be so, but... Granted, but... Personally, I wouldn't go so far as (to say) that.