I know I had a theory in the past where I said that Twilight Sparkle shouldn't be the element of magic, but it's evolved. I've got it figured it out, it was in front of us the whole time. Oh Cadance, how could we have been so blind?

Think about it:

Pinkie pie brings the laughs.

Applejack is honest to the core.

Fluttershy is so damned kind.

There's a theme, there. Their element is... well, their element. When fluttershy is being kind, she's in her element.

Let's review the past of Twilight Sparkle, the element of "magic".

When studying for magic, she had a difficult time and had to hit those books hard. She could barely hold a candle up to anyone else in terms of magic. Magic is clearly not her element, she passed that test into the special school by a fluke.

Fast forward a bit when they obtain their elements of harmony. Twilight sparkle didn't know what the last element was, so she bullshitted her little speech and guessed. She guessed wrong, but she got lucky.

Fast forward a bit again, Q Discord terrorizes equestria and steals the elements (oh no). During that crisis he corrupts the ponies to act the opposite of their elements. Applejack doesn't ever tell the truth, fluttershy stops being kind, yada yada.

What does Twilight Sparkle do? She keeps using magic. ("Here comes tom")

If she was truly element of magic, she should have become anti magic.

Fast forward to when best pony gets introduced

Shining armor keeps that shield up, until he doesn't.

Surely the element of magic should be able to magic a new shield right away, right?

right?

there's no element of magic, haven't you been reading?

Same thing immediately after with the crystal empire.

Time and time again, she's always shown studying magic because without intense work and dedication, she isn't very good at it. It's obviously not her element.

Season 4: each pony is tested on their element. They face a hard decision to own up to the element time and time again. Applejack is pained to tell the truth, but she does it anyways.

In the season finale, she gets overwhelmed by having the other alicorn magic. She should have been able to cope with that, but no, it's hard for her and she gets pretty out of control.

Her test would have been to use her magic, if it was her element.

But no, she gives it up.

She was tested on not having magic, which doesn't line up with her being the element of magic.

So, now that we have established what she isn't, let's dwelve on what she is.

It's really been in front of us the whole time, right from the very start:

My Little Pony: Friendship is Magic

When she calls upon the elements for the first time, she talks about how great her new friends are.

During the discord debacle, she shuns and discards her friends.

For her season 4 test, she makes the hard decision of giving up everything for her friends.

She's the princess of friendship.

Her cutie mark is literally herself surrounded by her five friends (spike isn't a friend, thanks for the confirmation hasbro)

She is: the element of friendship.