I guess i'll take on that last one, round out the list.
Rainbow Dash. Pure, adulterated, weapons-grade awesome. She knows she's the best, and she will not shy away from telling anyone who will listen. she's loud, she's brash, and she will not rest until the entirety of Equestria knows her name. You have to admire a pony with an ambition like that.
And that's not to say she's all bluster; she absolutely is not. She's performed a Sonic Rainboom, a maneuver that no other pony in the world has ever done, and she's done it THREE TIMES. if you do something that no one else has ever done, you have the right to be cocky.
Rainbow Dash is also very eye-catching; when "Rainbow" is literally your first name, your'e going to attract some attention. which leads me to her most important trait: her accessibility.
Rainbow Dash may just as well be the face of the entire FiM franchise. This may lead her being branded as the 'Mainstream' favourite , but I say anything that gets people talking about ponies is good for me. Chances are, if you're going to show a non-brony why this show is so great, you're going to show them Rainbow Dash.
I have a few more things, but I want to leave some room for others to share their thoughts.