Jupiter is the largest planet in our solar system, but CT Chamaeleontis is the largest known planet.

im not sure if it is the biggest in the universe but in our solar system, Jupiter is the biggest because you can fit about 1321.5 earths in it.

It is very hard to discover planets outside our solar system and pretty much impossible to discover outside the Milky Way. The largest planet ever discovered is CT Chamaeleontis which is estimated to have 17 times the mass of Jupiter. In fact it is so large it will probably end up being classified as a brown dwarf star. At that time one of the WASP planets (not sure which one) will be the largest known planet.

Here is a comparison of CT Chamaeleontis with Jupiter.