Princess Celestia seems the obvious choice, since she's run Equestria alone for god knows how long. But, I think if Luna was given a chance, she could be a good ruler. She's still young, and feels overshadowed, I think, but as long as she has learned that the night and day are both important, and doesn't go power hungry again... I think she could be better than Celestia, even.
Don't get me wrong, Celestia did a good job, and still is. But she just sits on her throne too much, and day by day, she's been getting older, less involved with Equestrian affairs. But Luna, while she may be more inexperienced, she's younger and more ambitious, has that passion and drive that could improve Equestria a ton as long as her 'improvements' aren't having anything to do with eternal night.