Web service getting value using LINQ from queue table in SQL database
LINQ is a good choice. You're querying something, so it lends itself to the task.

That being said, the exception handling is atrocious. Making your return values null, some command's value from the database or some big honkin' exception text is abusing the string data type. I'd remove the try..catch altogether and let your client app handle the exception if it knows what to do. In the best case, your catch could log the error somewhere then re-throw.