It really depends on the type of King you are looking into. Some are more relaxed than others, and some have a "bite first, ask questions later" type of attitude. California kings tend to be a bit nastier by reputation, but some are absolute sweethearts. I have 2 Cal Kings, and they have their moods, but are mostly very tame. My Desert kings are absolute dolls... I can't recall a time that they've ever turned an aggressive eye at me. My MBKs can be utter hellions at times, but I've worked with them relentlessly to tame them down with pretty good success. For the most part, kings and milks tend to have similar temperaments, though I've found in my experience that milks are the big muskers, kings are the big nippers.
Generally speaking, corns are probably your best bet for a docile pet. They too are individuals though, and some can be just as nasty (if not worse) than any king or milk. The key to all temperamental animals is time and hard work. They are ruled by instincts, and act quickly on them. If you can teach them that their defensive instinct is not necessary when it comes to you, then they all make wonderful pets. I wouldn't trade my nasty kings or milks for anything.