If you're in the market for an XML IDE with advanced validation, schema modeling and smart editing abilities, Oxygen XML Editor is a top pick. It's designed to support XML authoring and development with support for multiple schema types, smart XML editing and a tree-based outline view. It can be used as a standalone tool or plugged into Eclipse, and it offers advanced content completion.
Another top contender is Altova. This suite includes XMLSpy, a professional XML and JSON editor that can be used for modeling, editing, transforming and debugging XML and JSON documents. It also includes tools for generating reports and forms based on XSLT stylesheets, which can be useful for formatting and transforming XML data. Altova offers a wide range of tools and services, including data integration and contract management, so it's worth considering if you have a variety of XML and data-related needs.