Give your vote for Kayako, took her when she was a eSupport and the price was not so exorbitant, extremely intuitive and advanced system.
As far as I can tell, may all that you gave in your list. It is easy to set and then "just works", we have no problem with this system was not.
And terrified you here
. eSupport few years ago cost the company about 300 — 350 USD and from what I remember this is no limit on the number of users.
A drop of about OTRS, prakticheski this system is not used, but set "look", in my opinion intuitiveness compared to Kayako simply no, and to configure OTRS is another story.
A little bit about the Request Tracker, the same almost did not use, just install and explore the possibilities, be placed and configured much easier in my opinion than OTRS, for example, but the intuition is somewhat lame.
And by the way, Kayako doesn't pay me anything for such rave reviews :(