It is common and flattering to tell a man/woman
You're the most beautiful man/woman I've ever seen.
The thing is, this is never true. It's just like calling someone a "best friend". In most cases, people have multiple friends, all of equal importance, all "best friends".
For people, there are so many beautiful people in the world that singling one out as "the most beautiful" is a hyperbole. However, the effect of saying
You're the most beautiful man/woman I've ever seen.
Is profoundly less romantic if you say, in an attempt to not 'lie',
You're one of the most beautiful men/women I've ever seen
I need a way to rephrase this sentiment such that it implicitly acknowledges the untruth of the notion of a singularly "beautiful" person, without diminishing the truth of feeling behind the statement "You're the most beautiful man/woman I've ever seen".
For example: There have been several women, at various stages of my life, who have so enthralled me that I consider them all among "the most beautiful women I've ever seen". I knew I would be lying if I told a current partner, "You're the most beautiful woman I've ever seen", since this neglects the truth that I in middle school there was someone else I felt the same about, and her occupation of this position in my memory is not irrelevant, and to ignore it would be untruthful.
The assumption here is that anyone who thinks their partner is "the most beautiful person in the entire world" is not owning to the truth that they likely will encounter other people they would feel the same way about.
Best Answer
I disagree that the variant "You're one of the most beautiful women I've ever seen" is profoundly less romantic than the original. It's honesty is actually refreshing when compared with the very cliche original sentence, which might actually make the variant more romantic.
Further, this rephrasing seems to be precisely what you need to accommodate the truth of the situation, since truth seems to be important to you.
If there's any left-over sentiment that you need to capture, you might add a conjunction: "You're one of the most beautiful women I've ever seen and...".
Or, you could drop the quantifiers entirely and just go with "You're [insert poetic adverb] beautiful." For example, "You're howl-inducingly beautiful."