I'm fond of battlefield assassinations via mass-casting Ghost Riders when my Astral Windows or spies tell me a hapless AI pretender is out in the middle of a field away from a castle. Seriously, 'tho, Ethereal Stalkers are pretty nice if you can get them, albeit Chuzrael the Abysian Demonbred Slayer is much more entertaining.
A Heliophagus with a Black Heart and decent kit can also be unusually menacing, although not too stealthy IIRC. Black Hearts on high-stealth Glamour units could make for nigh-undetectable ones.
And if you allow mods... the Ashikagan Jonin, which I'm fond of prophetizing.
Re: Favorite assassin
Threaded Mode