Your question about "which school is best", is a minefield for Realtors. There's no way I could know what is important to you and your family without meeting you and having this discussion. I have sold homes to people who thought SAT scores were the be all and end all in determining the quality of a school, while the next week a different set of clients wanted a school that had football and after school band. So the question that I ask is,"what makes one school better than any other?"
The following link should help you with your delemma: http://www.greatschools.net/modperl/go/NH. You will need to copy the link and paste it into your search engine, but I think you will be happy with the results. I would like to magnify the answer Jerome Duval, left for you--if YOU are concerned with the quality of your son's education and you show that to him, he will get a good education no matter where he goes. Parental involvement is the trump card to a good education.
I'm sure moving to Manchester seems a little scarey, if you have lived mostly in a small town--but you will love New Hampshire. It's a great state in which to raise kids.