I beg to differ
Definition 1 of 1
Pro Tip 1/3Polite Disagreement
Use when you oppose an opinion but want to show respect for the other person's view. 
The movie was fantastic! - I beg to differ, though, I found it rather boring.
Pro Tip 2/3Formal Situations
Best used in discussions where politeness is important, like in a professional or academic setting. 
Our strategy is effective as is. - I beg to differ; we need to adapt to new data.
Pro Tip 3/3Slightly Formal
Although polite, it's less formal than 'I respectfully disagree', appropriate for casual conversation too. 
This is the best form of government. - Well, I beg to differ.