I'd say T'pring. Amazing presence & charisma without saying a word; stunningly gorgeous, memorable costume, hair & make up. Even her few lines of dialogue were perfect in keeping with her regal Vulcan disposition
Everything about that episode was perfect from the sound design to the rituals and everything on planet Vulcan itself. I think out of all the one-off Trek characters (before she was brought back in SNW) she made the biggest impression on me