Andrew Mitchell - He should have said nothing!!
There appears to be a lot of concentration on what he said. The fact is when asked by a Police Officer to exit a secure area via an alternative gate he should have done so. And if he felt he should say something it should have been "thank you" but really there was no need to say anything!!
A Police Officer will often receive instructions in a security situation about how members of the public should be directed. In this instance an Officer instructed a senior member of the Government to use an alternative gate. The normal response to this should have to been to obey the Officer on duty. If this were a Saturday and near a football ground I have no doubt that Mr Mitchell would call any refusal by a member of public to obey the Officer as yobbish.