You said main six which means all of them not just these two knuckle heads.

who cares "King-Sombra" and its mane not "main"

@Applejackrocks A MANE is the hair on ponies, also who cares? The question says "six" it does not say two of the main six.

All six should be options, obviously. People should at least be able to add them. Why only offer two choices unless there are only two competitors? Applejack would stand a good chance, and Pinkie...well, she's Pinkie. Anything could happen there. You might even imagine Rarity or Fluttershy winning in some way. Doesn't matter whether you write "mane" or "main," either. "Mane" is just a pun. ;p

At any rate, it's tough to say. Dash and Twi are definitely two of the strongest contenders for victory if they're all battling each other...

Twilight can reverse gravity , banish an Ursa Minor and I'm sure she's got lots other magic spells .
Rainbow can create a Sonic Rainboom and has super speed that can create tornadoes
Who knows who will win?

Twilight would win. She could disable RD's wings with her magic, RD would have no other way to fight her ( well she could use her hooves but who cares? )

In S2E3 Twilight stopped RD by using magic to hold her tail, and RD couldn't even move! So I choose Twilight.

To be honest, I think if they all got in a huge fight, the favor would be heavily in the unicorns' (Especially Twilight's) grasp. With not being able to basically do anything, Rainbow would clearly win.

Are you kidding me?

Twilight is a freaking ALICORN. Instant win.

Wow....