No no no and once more no! Twilight would never do as a good president. I wouldn't like to see president who, when not knowing what to do, would search through books and eventually ignore everything else until finding an answer in there. Moreover, she'd be TOO predictable. AJ is probably best option, since she seems to be using her common sense and logic the most out of all of them. Rainbow, well... yes. Rainbow. Let's move one. Pin~ Rarity. Only as a face of country. But even this would be hard, considering how fragile can politics be, and how easily you could offend her by an accident. And Fluttershy, like Rainbow, stay away. She couldn't finish anything she'd start if anybody even mentioned "Wondering" if they could refuse the idea. Although you can always count on Discord yet.