Complimenting someone's appearance should always be done with sincerity and respect. When telling a girl she is beautiful, it's important to be genuine and avoid using clichéd or overly cheesy lines. Here are some thoughtful and sincere compliments you can use:
"You have such a natural beauty that shines from within."
"Your smile is absolutely radiant; it lights up the room."
"I love how your eyes sparkle, they are truly captivating."
"You have an elegant and graceful presence that draws people in."
"Your style and confidence make you even more beautiful."
"I admire how you carry yourself with such grace and poise."
"Your inner beauty matches your outer beauty, and that's rare."
"You have a unique and mesmerizing charm that sets you apart."
"You're not just beautiful on the outside, but your kindness and warmth make you even more attractive."
"Your beauty is timeless, and I feel lucky to be in your presence."
Remember, compliments should be authentic and tailored to the individual. Appreciate specific qualities about her that genuinely stand out to you. Being respectful and sincere in your compliments will make a more meaningful impact and show that you genuinely appreciate and admire her beauty.