What's the best middle to upper class neighborhood in Austin?

If your budget for a house is 700K and higher than you can consider Allendale. Central Austin is even more expensive. If you can afford 400K-500K then NW Austin or Circle C. Otherwise, if your budget is skimpier, then you'll have to look at the Suburbs.

North Shoal Creek

How are you defining "middle to upper middle?" What income or net worth are you talking about?

Cedar Park is not upper--just middle-middle and very "suburban".
Try Northwest Hills, Allandale or Tarrytown

Not sure if this helps but I prefer the North side off Hwy183. Lots of nice areas here with good school districts. Also a little further out in Cedar Park, also very good schools.