Patrycjusz Malicki
Patrycjusz Malicki voted up Ray Dart's answer

The largest non-variable star, whose size we can be reasonably sure of, is VY Canis Majoris, but there are almost certainly many in the Milky Way that we cannot see (or whose size we cannot determine) that are larger. Wiki lists UY Scuti  as the largest, but this assessment is the subject of some controversy.