Limiting the choice to the shows and movies I have seen/know of. I'm not asking who the first actor you have seem playing Arthur was, rather the one you instinctively think about when someone mentions him or the one closest to your heart.
"The show is a medieval comedy one. This version of Arthur is trying to be a good king and to obey the Gods -read, find the Graal. He founds the Knights of the Round Table. His knights are laughingly incompetent. Karadoc and Percival are excessively stupid -but goodhearted-, Yvain and Gauvain are cowards and not very bright themselves, Leodagan doesn't like Arthur's modern sensibilities and only dreams of going to war, many don't know how to fight, Merlin sucks at magic"
"I think it would be funny for the new king of Britain to be French."
Stabby the Roomba
"A roomba with a knife taped to it. Subject of much lore about humans in space and their ability to bond with anything"