FreshDesk
Currently using FreshDesk for Internal support and hating it. The most basic things don't work. I spend more time fighting it than doing work. Its like they copied MS and sack their entire QA / usability team. Too many basic features not implemented and you pay through the nose for the higher packages that still sucks. FreshDesk project management component can die in a fire.
Too many things to list off the top of my head. Complete pain to correspond privately (not having the customer see the corespondent) with a third party troubleshooting an issue, forget it! You can do an initial forward but can't keep conversing on that thread with the 3rd party.
Want to link 10 tickets to a "Problem / Incident" . You would have though you can tick the list of tickets then assign them all to the "Problem". Congratulation you have to open each ticket individually and then link them from there.
The FreskDesk "Solutions" / KB article suck balls too. Formatting is so limited. My biggest issue is that you create a kb article and realise you got a typo in the title and fix typo or you decide to rework the title. Congratulation your URL to that solution is now broken! (it auto change the URL based on the title) No way to keep the old URL.
Zendesk, have used in the past. Wasn't too fond of it but didn't hate it.
HelpSpot: Not ITIL compliance but very easy to use. Designed by someone who had real help desk experience. Don't have some of the fancy features / integrations. However works very well for the feature it provides. I would use it again if I have any say in a helpdesk package. It has an API for you to integrate with other stuff. HelpSpot has Public Notes (aka customer corespondent), Private Notes (internal corespondent), External notes (corespondent with 3rd party). They have a policy of keeping it simple.
Managed Engine (I think it was): have used briefly in the past. ITIL compliance, UI was way too complicated and the 1990 wants it interface back!
BTW currently using Labtech but not even bother with its ticketing system.