"You Look Incredible...For Your Age"
Ageism detected. What does this imply? That the beauty of a person, and, more often than not, a woman, is solely determined by youth. Steer clear of this deep-rooted prejudice in the collective unconscious, which continues to herd people towards plastic surgery.
"You're Looking Great Today"
This compliment is slightly less barbed in as much as it might simply signal your appreciation of what someone is wearing at a given time. However, it might become unwelcome if it is repeated several times in the same 24 hours. Moreover, it often prompts the reply: "Why, do I seem ugly the rest of the time?"
"Have You Lost Weight?"
Here once again, context is key. In today's society, it may be considered judgmental to suggest that someone is more attractive because they are slim. If you are still not convinced, just think of the adjective "fat," which is so loaded it is almost never used. Just by bringing up the question, you are raising the issue of weight discrimination, which is why this compliment is best set aside.