I don't really go by rules in general. I go by policies and/or guidelines guided by my personal philosophy. Those "policies" and "guidelines" are my own, and they're sound because I've been a grown-up for decades now; and a "half-decent person who tries to do the right thing and doesn't want to hurt anyone/anything" even before reaching adulthood when I'd reached the age of establishing those policies and guidelines and a philosophy.
So, I guess my "one rule" would be to stay true to myself while trying to follow the policies, guidelines, and philosophy that both contributed to who/what I am as a grown-up but also where shaped by person that I am.
My thinking about rules is that there are two kinds - the kind that serve a purpose when a group of people are involved (as in organizes, companies, institutions, etc. - even individual family's homes) in keeping order in general, preserving respect for individuals in general, and whatever other purposes rules serve); and then there are "rules to live by" which, maybe more often than not, are the rules put together, taught, by others (living, dead, in our individual lives or not) that we then decide to blindly follow and/or selectively follow or not.
There are a whole bunch of other people's "rules to live by" that are good rules. In fact, they're good enough that if/when a person reaches a good, solid, maturity/thinking "level" will very much recognize as "a good rule" and adopt as his own. There's a reason so many "rules to live by" have remained as "good" or as popular as they have for so many years throughout history.
When it comes to "personal rules to live by", however, the person who, say, gets to be twenty-one and thinks that's all there is to "good living" and/or happiness and just goes on with the business of living his life will a) either live life in a kind of "robot" way, b) live life in a self-righteous/kind of arrogant way, and/or c) possibly learn more about the differences between rules, policies, philosophies, etc. as life kicks in and he gets a little wiser about things.
When life gets complicated and there are people to consider and understand beyond just our little selves (and life tends to get more complicated when, say, we have a family we care about, for example) some of those "rules to live by" that came from outside us can turn us into destructive people and/or themselves become destructive. These shallow rules that sound good and that we memorize are far from adequate.