roxanne.....I LIKE this question. Having raised 4 sons, you can be sure, I gave all of them, an entire boat load of advice on "women."
I'm afraid that in terms of "the most IMPORTANT piece of advice" about women I gave each one, they would probably have to tell you. Because they are all such unique individuals, my guess would be, that each one of them chose their own most important morsel of advice.
However, I can assure you that I continually reminded them of the vast worlds apart, in the way men think, as opposed to women. I encouraged them to always remember that above all else, women need to know they are LOVED & APPRECIATED......not "now & then," but at all times. That, no matter how strong, self-motivated and independent a woman is....she will always want to know that the man in her life is her safe haven and a soft place to fall. She will always need for you to be her Hero....her "Knight in shining armor," who charges in to defend and protect her, if necessary.
When women confuse and frustrate you (and they will) keep in mind that it's less important to "understand," than it is to simply love and respect her.
Since the reality is that, simply by nature, women are all about love and nurturing, there is never a need to make irrational demands. Treat your woman as if she's all important to you and you'll receive it all in return, a hundred times.
"Intention," as well as words & actions, tell women how you really feel. Practice the common courtesies you've been taught since being babies.....words like "Thank you", "I'm sorry", "excuse me" and "Can I help you?" are powerful. Use them often.
This is pretty much the basic course I taught in, "Women 101." I will probably never know just how much of an impact any of the lessons had on them. But I can very happily tell you that all 4 of them have visibly strong, happy marriages, to women they love dearly and who love them back fiercely!
As a little side note, I will share a special blessing I've received that makes my heart sing. At one point or another, I received a personal letter or card from each of my daughters-in-law, in essence, thanking me for raising an amazing/awesome man and how lucky they feel to be their wife. Seriously, I can tell you, I literally framed each note and hung them on my walls. My 1st. DIL signs all cards to me..."Love, Missy & The Love of my Life." They've been married nearly 25 years. Am I a blessed Mom? You bet I am!