Whatever rate maximizes economic output and government revenue in the long term.
It can't be too high because economic output will suffer and people will leave. I know tons of young professionals in Canada who went into medicine, engineering, law, etc., who are planning to leave Canada for America simply due to the fact that they will get paid more and taxed less in America. They get their childhood, medical costs and education subsidized by the Canadian government, then they are going to leave before they have to pay any of it back. Canada's rates are not competitive enough with America's and it's costing them big time. Luckily Canada has an influx of foreign immigrants willing to invest millions in the country, but that can only last so long and is really starting to hurt Canada.
It also can't be too low, because you then couldn't fund the necessary aspects of government that can only be effectively and efficiently done by government (military, police, firefighters, space programs, etc.).
It's not an easy answer and I doubt anyone really knows what rate will be best as there are simply too many variables to take into account. But we can look historically, and we can see that established economies generally tend to do better with less tax than more.