I am trying to build application to pull property information and make decisions based on the data available for the house, neighborhood, properties sold around the same are ect….What is the best fit company/API which can provide API data (XML/Jason) which does the following?

City-Data, Neighborhood Scout and Rentometer are other sites I look at.

Prices will always be driven by demand and supply.

They claim to have good data, but some of what you are talking about might not be available more often than the 10 year census so make sure to check the data sources.

If not, anyone know a cleaner data source I could tap into for this kind of research?

I don't have enough data yet to quantify how much, but it does work.Hope this helped!

My opinion is that tax assessor data is completely useless for establishing property value.  

Their data is woefully out of date and useless. 0-2 is enough for me.

I think its being by driven the shift toward urbanism and specifically the staff at the Hospital Center discovering the close proximity to work and all the amenitiesI'd really like to do some deals in the neighborhood to build my reputation in the community.  

@Kate Stallmann Not really enough data, but that's a very tight deal at best and it could turn quickly into a bad deal if there is any kind of deferred maintenance at all.