“What is the most serious type of heart arrhythmia?”

By far the most serious irregular heart rhythm is a condition called ventricular tachycardia. These are fast and have normal beats that come from the pumping chamber or the ventricle. in some cases these can degenerate into a fatal heart rhythm. That is the most serious. Atrial fibrillation is also serious heart rhythm abnormality when the top (atrial) chambers are beating very fast. This can predispose one to having a stroke and requires lifelong anticoagulation.

Serious arrhythmias are those that cause either a cessation of the function of the heart or a heartbeat so fast and disorganized that the heart cannot pump blood. In the first example,, cessation of heart function. Is called that asystole "no contraction". That could be a primary lack of electrical function. Think of the reason for a pacemaker.
The fastest and most disorganized ventricular arrhythmia is called ventricular fibrillation. The heart must always be a functional syncytium propagating electrical impulses between communicating cells so that the complex and varied components of the heart can act as a single contractile apparatus. When all the electrical communication doesn't work neither does the heart muscle

Typically, ventricular tachycardia, or tachycardia from the lower chamber of the heart, carries a higher risk than arrhythmia originating from the upper chamber. That being said, there are always exceptions to the rules.