Of course the Sun. But otherwise I would say either Betelgeuse or Rigel. Both are easy to see, bright stars at opposite corners of the constellation of Orion (Betelgeuse top left, and Rigel bottom right) but both are really impressive in different ways.
Betelgeuse glows slightly orangy in the night sky because it is a red supergiant, several thousand times brighter than the Sun, and only 10 million years old, yet soon to end its short life in a supernova explosion. Most impressively, if it was placed where the Sun is , we - and Mars, and maybe even Jupiter - would all be inside it!
Rigel is not nearly so voluminous as Betelgeuse. but it is much hotter and intrinsically brighter. Estimates vary, but it is probably 60,000 to 80,000 times brighter than the Sun. Impossible to imagine!
There are even bigger and more luminous stars but these are the two most easily seen.