Both versions.
Classic Usagi because she's funny and adds a lot of comedy to the show. She's the Usagi I grew up with. Although she remains immature during the entire 5 seasons (e.g. Mamoru gives her a ring in Stars, and she seems clueless that he proposed to her), she's funny. By the time that Stars ends, Usagi essentially still is a clumsy girl, and I find that charming. It's a reminder that we can be clumsy and silly and still be a super hero and save the planet.
Crystal Usagi because she's more serious, focused, and mature. She prevents Mamoru from kissing her when she's angry, has sex (okay, the scene wasn't as graphic as it could have been, but it's there), and rejects living as a Queen after the moon kingdom is restored. She is a kid and acts like a kid in the first episodes of Crystal, but by the time that the Black Moon arc begins it's clear that Usagi has become a woman, and is no longer a kid.