Which branch of government is the most important?
Neither branch of government is the most important. They all have duties that they must carry out and without those duties, the duties of other branches also can't be carried out.
None. They were designed to be interdependent.
If you're speaking about a government with branches like the US, all are equal in power. It simply depends on what you view as important.
none, they were designed as a triune setup for a reason. That reason being that all three are equivalently necessary in order to maintain the nation.