Twilight Sparkle: 7w6
The bookishness is not the whole story. She fairly patently enjoys her pony friends and being part of the group. She's far too emotionally expressive for Fe DS, but in a really awkward and adorkable way. The bookishness comes from needing to study in order to tame her raw magical power.
Rainbow Dash: 6w7/8w7
Interestingly, plays off Applejack. Both are loyal to their friends (which seems pretty E8), but Rainbow Dash is a boisterous and impulsive tomboy, while Applejack is down-to-earth and hardworking. Earlier episodes contrast them much more than later ones.
Pinkie Pie: 7w6
Hands-down the best character. If you disagree, shut up. You're wrong. That's all there is to say on the matter.
Princess Celestia: 9w8
Barack Obama in magical unicorn form.