Maybe "being your natural-self" ? ? One could argue that when they are themselves they are not attracting opposite sex like they would want, so then they try a different strategy - becoming someone that is less like them self but what they think is more "appealing" . This can work and most often will work, then when the "true" self comes out it can cause a problem...Maybe.. I think when someone says be your "best" self... you are focusing on your best traits... not that you are hiding your other aspects, but rather highlighting your best part of yourself - not in a boastful way, but a natural way... If you can make someone laugh, that is worth it's weight in gold and most people like to laugh... being able to laugh at yourself in situations you have found yourself in is probably even better.. it shows you are human and open enough to let them see what some would consider vulnerability. Not so "Macho" as some could say, but it is more attractive I would think to the opposite sex. I think the goal should always be to meet another person, treat them how you want to be treated and not force anything... at the end of the day, you may find you have met a unique person and you get along... or you find you don't click with them, but it was nice to learn that as well so there is still value in the interaction.
I hear others too often say that when they met they were perfect... then as time goes on the perfect wears off... could be true, or you just seeing things more clearer... Sorry, no scientific data to back up this opinion
that's why it's an opinion and not scientific fact
Accept who you are first, like yourself and then be the part you like the most. Try not to be insecure, this leads to jealousy, controlling and manipulative behavior which are far more damaging to a relationship then average looks... IMO....
Get to know the person... ask questions and listen to responses... you know "normal conversation"