You're one of the most beautiful lady/ladies that I've ever seen.

[Dharminder Singh]

You're one of the most beautiful lady/ladies that I've ever seen.

Which one is correct - lady or ladies?

 

[Piscean]

Ladies. She was either the most beautiful lady or one of the most beautiful ladies.

 

[emsr2d2]

Just remember that "One/two/three (etc) of the ..." is always followed by a plural.

 

[teechar]

You could also drop "that" if you wish.