Formal:
* Vous avez de beaux yeux. (You have beautiful eyes.) This is the most direct and formal way to say it.
Informal:
* Tu as de beaux yeux. (You have beautiful eyes.) This is the informal version, used with friends or family.
* Tes yeux sont magnifiques. (Your eyes are magnificent.) This is a bit more poetic and expressive.
* J'aime beaucoup tes yeux. (I really like your eyes.) This is a more personal and affectionate way to say it.
More poetic options:
* Tes yeux brillent comme des étoiles. (Your eyes shine like stars.)
* Tes yeux sont comme des saphirs. (Your eyes are like sapphires.)
Ultimately, the best way to say it depends on the context and your relationship with the person.