Greetings...
The simplest way to freak people out (however old they are) is to sneak up on them - and until I discovered this site, my memories of Knightmare were mainly of its "stalkers".
(How odd that the Automatum was only in one series: he, along with the skulls and later stormgeists, summed up the whole show for me).
I guess the scariest ones must be Series 3's Ogre (a stalker who actually shouts!) and Hobgoblin ('cause he was so huge). Having said that, I clearly remember marvelling at the sheer grossness of the Miremen, and jumping out of my skin when the Behemoth appeared. (For some reason, in the intervening years, I thought his/its name was Cleave! Right vowel sound, wrong weapon?).