Could be a sexual act or moment such as when she is gazing at you from below or bent over with an arched back.  Possibly its in the early morning as she is stretching or when she is discussing something she is passionate about.  

Confidence and self-assurance, tempered with humility. A great sense of humor, including the ability to laugh at herself (and not take herself too seriously).  Frequent unforced smiles, and laughter.  Eyes bright with energy and intelligence and love.  Being engaged in the moment, whatever it is.  Being kind towards others, especially when there's nothing in it for her.  A positive attitude towards Life.

when she is very clearly present--doesn't matter the pose or act, as long as she's present and attending to the moment, it's a moment of intimacy, and for me, that has to be there for a woman or situation to be sexy.

have, through genuine compassion, tenderness, mutual trust and honest rapport ... broken down her defenses ... and she has given me her "EVERYTHING". When in such a state, I think the angelic face she makes while we are joined "as one" is the sexiest thing I've ever experienced ... hypothetically.

It is that moment when the session is over. It is when the clothes are back on and you are getting ready to walk out the door. It has happened a couple times that when I make that final eye contact, there is something in her smile/eyes and the tilt of her head that says "you will not forget me."  

Sailana,

Knowing that this topic is very much subjective, I would like to discuss the difference between attractive and sexy. I see confidence as sexy, whereas intelligence I see as attractive. When a woman is able to inform me of an interesting theorem, I am not turned on, but if she is confident in the way she presents it then I am turned on. However not mutually exclusive, I believe there is a difference.

All the reasonings of men are not worth one sentiment of women. Voltaire

Whatever floats your boat!  Personally I don't think I'd get turned on by a woman presenting a theorem confidently.  Although I did think that this coed I knew in college and was great in math was pretty hot.  

At least they smell the sexiest at that time... And also seems to "act" more sexy and responds in a more alluring way...

Being a woman who's thoughts are greatly ruled by emotions, I agree he said it well.  Thank you to all the men who truly appreciate women, and all of our charms. Here's another worth sharing:

she gazes into my eyes and says....

his wallet

He is a good father and treats his woman well. For me foreplay can last all day; all the lil sweet things and compliments he does makes me want him. He can dress up yet be comfy in jeans and tshirt. I also think the backside of a man is the sexiest (back and ass).

...whether or not she tries to be. It can something very candid like at the gym when she's seriously working out on the elliptical with sweat moistening the seams of her sweats and I'm fortunate enough to be watching on a machine behind her to seeing her dressed in her little black dress out in public weathering the admirable gazes of the lusting gentlemen and the critical glances of her female competitors. There's an unending list of when I feel a woman looks the sexiest but, I know one thing, a woman never looks sexier than when....

She is all over me ... Plain and simple

There's an ebb and flow to the time a man and a woman spend together and a lot of different variables that have to peak at the right moment.  It could be in the midst of one activity one time and something totally different the next.  It's not random as some moments are far more or less likely to produce that peak but at least for me it's not something I find very predictable.

Easy question.  When she walks toward a goal or an appointment.  When she knows men's eyes are following her and she struts quickly on her high heels, eyes straight ahead and posture....perfect.  A strand of hair falls on her cheek and instead of brushing it aside, she moves her head in one slight shake.  The Vera Wangs she has on clips onward as if to say to everyone, "Take a good look, people, you won't see anything like me very often."  Men watch her fade in the distance as they notice their member is hardening.  Quickly, they think of the Super Bowl or the repairs they need to make on their car.  They must force the picture of that Godess out of their mind, pretend it was a mirage, a dream...but they're only kidding themselves.