Explanation:
The executive. Of course it will depend on the country in question.
The Executive controls the levers of power. The other branches can write and judge compliance with the the rules, but they don’t actually have any agencies with whom they can implement their decisions.
In many nations the judiciary and the legislative branches are very week. For example in communist nations the Parliaments were rubber stamps which simply did what they were told and the courts were packed with friends of the state.
We also see what happens when Parliaments become too powerful. In Germany the Wiemar republic was able to throw out the government very easily and this lead to a lot of instability and eventually the total removal of the government. When Germany was reformed after the war they changed the rules to make things a lot tougher to throw out governments. In the US the President has fixed 4 year terms and is very hard to remove - otherwise the President would be dumped every time the other party won the 2 yearly mid term elections.
When the executive branch teams up with the Legislative Branch - as in a Parliamentary government system, the executive branch becomes even more powerful. Especially if the government has a strong majority in Parliament. The strong support for the Government by Parliament allows the executive to do a lot of things in contrast to say a Congressional system where the President has no control over the congress even when held by his own party.
In the extreme cases of New Zealand and the UK, the Judicial Branch has very little ultimate power at all. This is because the Constitution of these nations is set by ordinary statue law. Thus any judgement of the courts can be easily overturned by the Parliament at will. For example the courts said Brexit could not be initiated by the PM. Parliament passed a law then saying the PM could start Brexit.
Contrast this with the USA where many times the US Supreme court alone has determined certain things are legal, such as slavery, abortion and homosexuality without having these decisions overturned(But slavery was later banned by constitutional amendment).
In the English Civil war we had a occasion where the strength of Parliament versus the Crown (executive) were challenged by battle. Parliament won, however the leaders of the Parliamentary side eventually found Parliament too much trouble and got rid of it.
The Strength of the executive Branch declines and grows with the rule of law. Often nations with good rule of law a richer and thus the executive Branch can do more stuff because it has more money. On the other hand rule of law reduces the power of the executive branch because the fact that they must follow the law.