I would have to say Houston is probably the largest but best is subject to opinion
http://www.quilts.com/quilt-festival-houston.html. Paducah is probably a very close second. After that any of the AQS quilt week events are fun and there is usually one held within driving distance for anyone in the country.
Then there are MQX, Road to California, Sisters outdoor quilt show, The Mancuso Quiltfest events. there are so many.