This article is concerned with radius of the star rather than its mass. If you were to do a search on the internet for the answer to this question, you wouldn't get a definitive answer as you would if you asked 'What is the largest planet in the solar system?'. Which the answer to that question is of course, Jupiter. If however you did 'What is the largest star?', you will get a selection of answers. Some will quote VY Canis Majoris, other sites will champion NML Cygni and then finally others of which I am one will say that the largest star is UY Scuti. Why the discrepancy, well, if you look at the dates when the articles were put on the system and then look at when UY Scuti was announced as the biggest star, 2012. ref:wiki You'll understand why the discrepancy. The more recent the article, the more likely to say UY Scuti.
All those and plus a few others are gigantic stars, many thousands of times bigger than our own Sun. UY Scuti is rated at being 1,700 times the size of our own Sun. If UY Scuti was placed at the centre of our own solar system, the Earth would be inside. UY Scuti reaches to out past the orbits of Jupiter and even Saturn. These stars are all red hypergiants, most likely at the end of their lives at a point when nearly all their fuel is used up and is about to explode. Most of these stars are variable stars, these are stars that pulsate, their size changes over time. For example, VX Sagittarri, another hypergiant star over a period of 737 days according to the Hipparcos Star Catalog.
The universe is so vast, there could well be a star that would make UY Scuti look insignificant as UY makes the Sun. We have just not spotted that star yet. The problem with star size is that the larger the star, the shorter it lives for. Our own Sun has a lifetime of 10 billion years of which we are half-way through but Proxima Centauri, a small red dwarf star which is a mere 4 light years away and our closest star neighbour is expected to have a very long time, one that could be trillions of years. If we ever leave Earth, we should hope for a hospitable planet orbiting round Proxima to continue the human race.
We may never find actual biggest star in the universe because it is so vast and that there are so many galaxies out there that we couldn't measure all the stars in each galaxy. For the time being we should be content with UY Scuti as the largest for the answer until a bigger star is considered. A special mention should go to Betelgeuse, in the constellation of Orion is a giant, not as big as the others but still big and is expected to go supernova in the next million years. When the other stars will go pop so to speak, I haven't discovered anything yet.