Reputation on StackOverflow is supposed to be as strict an approximation as possible of your technical contribution to the site. Anything along the lines of "Most useful" is not, strictly speaking, a technical question (especially if it ends with "rule of thumb"), because there is no right answer to the question; it's a matter of opinion. Other questions may also have more than one right answer, but whether or not those answers work is not a matter of opinion; anyone can test them and see.
Your question definitely sets up a poll, in that people will vote on answers in the same way they'd vote in a political election or respond to a political poll. A side-effect of Community Wiki is that is does not give reputation, and so it can mitigate the issue somewhat. Nevertheless, such questions do not really belong on StackOverflow itself; it's not what Stack Overflow is about.
Occasionally a question such as yours will bring in so many good answers or be so useful that it manages to succeed on Stack Overflow anyway. I have one like this myself, from the early days of Stack Overflow before the rules were as clear. These questions are the exception. They are akin to winning the lottery (and hopefully you're smart enough not to play that, too).
Recently there has been an upswing in subjective questions on Stack Overflow, thanks largely to this post on the Stack Overflow blogthis post on the Stack Overflow blog. Near the bottom of the post there is a collection of six guidelines for a good subjective question. Yours fails on at least three points (#s 2,4, and 5).