Do *not* use ManageEngine ServiceDesk Plus. I'd also be quite reticent to try any of their other product. SD+ is remarkably awful, especially in its email integration, but it's quite bad in pretty much every aspect I've found.

I've used RT before, and it's very configurable, but it was a nightmare to set up initially. That may have improved in the intervening years, but I doubt it, as it intentionally had no defaults.

You might also want to look at Wikipedia's Comparison of Issue Tracking Systems.