<p>We have developers with knowledge of these languages - Ruby , Python, .Net or Java. We are developing an application which will mainly handle XML documents. Most of the work is to convert predefined XML files into database tables, providing mapping between XML documents through database, creating reports from database etc. Which language will be the easiest and fastest to work with?
(It is a web-app)</p>

In .NET, C# 3.0 and VB9 provide excellent support for working with XML using LINQ to XML:
LINQ to XML Overview