Formal/poetic:
* L'amoureux de mes rêves (literally "the lover of my dreams")
* Mon amant rêvé (literally "my dreamed-of lover")
More casual/colloquial:
* L'homme/la femme de mes rêves (literally "the man/woman of my dreams")
* Mon/ma rêve (literally "my dream" - this is very informal and suggests the person is an idealized fantasy)
More specific:
* Mon amant idéal (literally "my ideal lover")
* La personne idéale pour moi (literally "the ideal person for me")
The best option for you will depend on the context and the tone you're aiming for.