Since this has been bumped, I’ll go ahead and say that my favorite ET of H3 has been…
Probably most for its dark humor and the setting/situation. They’re not too easy, but certainly not impossible.
As for a challenging ET, I think that’d be The Ascensionist.
The Liability was probably the easiest*. But funny writing for the script.
Edit: “Easiest” - but I essentially would’ve lost SA because the gal escorting him through the place caught up and was seconds from walking through the door to spot me dragging his dead body. 
So I wussed out and Alt+F4’ed.