Hi Brian, It depends on what aspect of the storm you are using to decide if it is the strongest. One possibility is the highest number of high energy particles measured during the storm - although, even there you need to specify what energy of particles you are looking at. In those terms, the biggest storm directly measured was probably in August 1972.
Another measure is how the magnetic field of the Earth is temporarily affected by the storm. Although, this is before accurate measurements were made, there are historical records that suggest the largest storm was in September 1859.
Finally, one can look at the energy of the x-ray flare. The affect of the flare on Earth depends on whether it was directed towards the Earth or not. The largest flare on record occurred fairly recently - November 6, 2003. You can see a picture of it here: http://sohowww.nascom.nasa.gov/hotshots/2003_11_04/ -Christina