My first guess would be Rhododendron subgenus Azalea indica 'Mrs. G. G. Gerbing'
http://www.monrovia.com/plant-...
There's also a more compact Southern Indica Azalea called 'Fielder's White' that has similar flowers but I'm not familiar with it. 'Mrs. G. G. Gerbing' gets really big so that might help some in the ID.
