I my IT career I have used soooooooooooo many Helpdesk systems and by far I found the winner has been OSTicket, it is simple, full featured and really just works once configured.
I used to run RT, OTRS, ManageEngine, Salesforce and others either they were to complex, to difficult for people to use quickly and just cumbersome. OSTicket does everything right, yes they could use some more features but at the end of the day the extra feature we just would not use.