Luckily we don't have to speculate. See a web interface at [1]. Select English from the dropdown and it says that the closest word to king is kings and second is queen, then monarch. And to test my second statement I also tried "red is to king as blue is to ___" and got kings and queen as the top two answers.

[1] http://bionlp-www.utu.fi/wv_demo/