It's tricky to definitively say which star is the *largest* in the universe, as we haven't observed every star. However, there are a few contenders for the title of largest known star:

* UY Scuti: This red supergiant is currently the largest star by radius, with an estimated radius of 755 times the radius of our Sun.

* Stephenson 2-18: This hypergiant, also a red supergiant, is believed to be larger than UY Scuti, with an estimated radius of 2,150 times the radius of our Sun. However, its size is still uncertain, and further observations are needed.

It's important to remember that the size of stars can change over time, and new discoveries are always being made. So, while UY Scuti and Stephenson 2-18 are currently considered the largest known stars, that could change in the future.