What is the best way to retrieve WorkItem programmatically? Is it by
using the IQueryClient interface by defining an expression?

It depends ;)

If you have a work item handle, you can use the ItemManager to fetch the
work item or alternatively
IAuditableClient#resolveAuditable

If you have a work item ID, you can use
IWorkItemClient#findWorkItemById

Otherwise, you have to query for the work items. For details, please see:
https://jazz.net/wiki/bin/view/Main/QueryDevGuide

--
Regards,
Patrick
Jazz Work Item Team