That's a very great question. Of the three, I would honestly say Cadance and Luna simultaneously. Twilight's the Princess of Friendship, and she said it herself in Twilight's Kingdom that this is a role she proudly chooses to have. She may be groomed to be her successor, but I don't think she wants to be a part of it. Luna and Cadance share roles that others rely upon for survival (Luna raising/setting the moon, Cadance protecting the CE). Speaking for Luna, her temper has been much more controlled since Luna Eclipsed and accepted the role as dreamhopper to help protect presumably foals in need. But given the rumor unleashed by the card game that she could be in another dream episode later this season, we may see more from her later.

That's a very great question. Of the three, I would honestly say Cadance and Luna simultaneously. Twilight's the Princess of Friendship, and she said it herself in Twilight's Kingdom that this is a role she proudly chooses to have. She may be groomed to be her successor, but I don't think she wants to be a part of it. Luna and Cadance share roles that others rely upon for survival (Luna raising/setting the moon, Cadance protecting the CE). Speaking for Luna, her temper has been much more controlled since Luna Eclipsed and accepted the role as dreamhopper to help protect presumably foals in need. But given the rumor unleashed by the card game that she could be in another dream episode later this season, we may see more from her later.

Very interesting points. Maybe Luna will rule from Canterlot while Cadance maintains her role in the CE and Twilight maintains her role in Ponyville.

As far as I'm concerned, Celestia is unfit to lead now. I'm still firmly under the belief that Equestria is in serious need of a new ruler. She lacks any ability whatsoever to keep her subjects safe, and in her last appearance, she nearly got each and every one of them drained of their magic while she blissfully depowered herself without any concrete plan in mind whatsoever. She is a joke now. 

I honestly can't say that Luna would be much of an improvement, though I think she would at least deserve a chance to try. And I would feel far more optimistic about Cadence, even though she's had her own fair share of failings. 

But Twilight is just the obvious choice. Over the course of one season, the role reversal between her and Celestia has just been....staggering, to say the least. Twilight has become the standard by which Celestia now fails to live up to. No longer does she need to be there to clean up Twilight's messes as she did in Lesson Zero. Now, Twilight is the one who needs to be there to clean up Celestia's messes. It's no question why her role in the show now is so nonexistent. Twilight simply has no use for her anymore, and the show has failed to provide a significant reason as to why she should continue making appearances. 

As you said, Twilight is the whole package. The "be all, end all." If the next episode had her being crowned as the sole ruler of Equestria, everyone would be much better off for it. 

As far as I'm concerned, Celestia is unfit to lead now. I'm still firmly under the belief that Equestria is in serious need of a new ruler. She lacks any ability whatsoever to keep her subjects safe, and in her last appearance, she nearly got each and every one of them drained of their magic while she blissfully depowered herself without any concrete plan in mind whatsoever. She is a joke now. 

 

I honestly can't say that Luna would be much of an improvement, though I think she would at least deserve a chance to try. And I would feel far more optimistic about Cadence, even though she's had her own fair share of failings. 

 

But Twilight is just the obvious choice. Over the course of one season, the role reversal between her and Celestia has just been....staggering, to say the least. Twilight has become the standard by which Celestia now fails to live up to. No longer does she need to be there to clean up Twilight's messes as she did in Lesson Zero. Now, Twilight is the one who needs to be there to clean up Celestia's messes. It's no question why her role in the show now is so nonexistent. Twilight simply has no use for her anymore, and the show has failed to provide a significant reason as to why she should continue making appearances. 

 

As you said, Twilight is the whole package. The "be all, end all." If the next episode had her being crowned as the sole ruler of Equestria, everyone would be much better off for it.

I'd agree that Equestria seems to be under threat quite a bit under Celestia's rule. Perhaps Twilight could restore the regime's monopoly on violence by keeping those pesky invaders and escapees far away from Canterlot. I'd say the one area Celestia has an edge on her is diplomatic relations with other rulers, like Saddle Arabia for instance. She has connections and commands the respect of ruling for 1,000 years that Twilight can't quite claim at this time.