It's tricky to definitively say what the "biggest" star is, as there are different ways to measure size:

* Radius: For sheer size, the current record holder is UY Scuti, a red supergiant with a radius estimated to be about 1,708 times that of the Sun.

* Mass: While UY Scuti is massive, it's not the most massive star known. That title goes to R136a1, a blue supergiant with a mass about 265 times that of the Sun. This star is also incredibly dense and hot.

So, depending on how you define "biggest", the answer is either UY Scuti (largest radius) or R136a1 (largest mass). It's important to remember that the universe is vast and we're constantly discovering new and even larger stars.