I have used Zendesk, Freshdesk and GrooveHQ. Zendesk scores the point because of its ease of use, its support, and an excellent knowledgebase platform to complement your support process. When you setup a help desk system, plan a few support articles in the Knowledgebase (Zendesk helps you here) to reduce the support calls.

I have helped business setup this support process, including the KB. Let me know if you need more details.