1. Tool which will allow stand alone testing of web services
2. Tool which can be used as a test harness to create different suites of functional and regression test cases.
There are other factors also which influence the selection of tool. I would like to prioritize them like this:
1. Tool allows you to create test cases and Test case Management.
2. Tool allows integration with other frameworks or applications.
3. A good result reporting format.
4. Tool supporting different platforms.
5. Tool should be easy to learn.