Loves books and learning, slight social issues, gets along with everybody... I feel like Twilight would seriously be the perfect match for me if she really existed. That's all I got.
For me personally, Twilight seems kind of boring. She's the
straight man of the group. That makes her very useful for the group dynamic, but personally...not very interesting. She's
adorkable, but Fluttershy is more lovable, and personally, "lovable geek" doesn't do much for me. "Loving books and learning" and "has slight social issues" ...I don't see these as positive traits. I'm sure Twilight would be fine to visit, would make a good ally, would be reliable, dependanble, and a good friend...but she doesn't inspire an emotional reaction in me like some of the others.
Have you ever flown faster than the speed of sound? Have you ever heard of any creature alive that can?
There is one pony that can do this by flapping her wings hard. She is best pony.
Ok, but in a world where a third of everypony can do magic, being able to fly fast somehow seems less awesome to me. More to the point though, how would you make a relationship out of "she goes fast" ? And maybe that question doesn't matter to you. Perhaps you'd be content to put her up on a pedestal and admire her from a distance as you squint to try and see her while she flies. But to me, I don't see a lot of potential for actually relating to her personally. Like TheBronzePickle said a few posts up, you could probably get to be her friend by feeding her ego, but again...it doesn't particularly appeal to me to try to make friends by telling someone how awesome they are.
When I think about "which is best pony" I don't imagine them as faraway entities that I'll never meet. I think of them in the context of actually spending time with them in person. Twilight would make a good friend, sure...but what would one actually do with her? Read books together? Umm, ok. And Dash, again, if you can't fly, the fact that she's a fast flier just means you don't get to spend much time with her. And if you can fly, can you fly faster? If you can, doesn't that diminish her awesomeness? And if you can't, is it really that much fun to have a friendship with someone based on the fact that they're better than you? Or are you aiming to be neck and neck and have a friendly rivalry? I can see that working, but when I hear people say Dash is their favorite, I don't usually get the impression they imagine competing with her at all.
if one were to actually have a personal relationship with a pony, rather than simply admire them from a distance, I think Pinkie Pie would be the most fun. Applejack might be the salt of the earth, but I wouldn't want to spend my social time collecting apples. Rarity is classy, but she's busy most of the time. Sure, you could offer to dig gems for her, and you could talk to her a lot, but whether in the shop or digging gems, eitehr way Rarity is a bit too much of a workaholic for my tastes. Fluttershy is adorable, but I think having to be delicate all the time would get tiresome eventually, and taking care of animals isnt' my idea of a good time.
But Pinkie, you could be magically transported to Ponyville and in five minutes be dancing and laughing and playing and having food fights and eating candy and cake. That sounds fun to me. And when crazy adventures happen, Pinkie would be there enthusiastically enjoying them. It might be hard to keep up. But it would be more fun than reading books, or picking apples, or watching the speck in the sky that is Rainbow Dash, or digging gems or standing very still while Rarity uses you as a mannequin, or cleaning up after animals.
Of the mane six, Pinkie would be the most fun.
Now, if you want shipping, I don't think Pinkie would make the best girlfriend. But if I were transported to Ponyville, of the mane six, she'd be the one I'd be most excited to meet and spend time with.