We use a Ball 21 quart on our glass top and it is fantastic. Fits about 7 quart jars (depending on wide-mouth / narrow-mouth). We love it. It was a bit pricey, but totally worth it to speed things up when canning big batches.
We also sprung for the stainless steel jar rack, since the standard ones always tend to rust at the welds after a couple of months of use.