While Applejack easily has the most brute strength, Rarity strikes me as the type of pony that would know kung-fu or something. Excluding unicorn magic, this is a case of strength versus skill. My money is on Rarity.
Rarity: "Yippee ki-yay, my dead, illiterate Applejack!" "Howdy, pardner!" "Happy trails, laborer!" "Did I throw a 'wrench' into your plans? (laughs maniacally)" "Not much of a quick-draw, are you, pardner?" "Giddyup now -- to hell!"
Applejack: Now just stop trying to mess with my contraptions. It ain't gettin' any harder for me to kill ya! You know where you oughta hide next time? Back in France. Dominated, you turncoat! Dominated, you snake! This skunk's startin' to smell. Slither on back to hell, Rarity! That's what my daddy taught me to do to backstabbers. You're a looong way from France, girl. Don't go meddelin' in my business. You are a coward an' a scoundrel. If y'all had more gadgets for killin', you wouldn't need so many for hidin'. I just beat on your sneaky ass like a mule, girl!
I love the way you do pony anatomy. Usually I have a hard time picturing them standing on two hooves, but you did a very good job of drawing them in their stances. I can see that AJ is a straight-up brawler type, while Rarity seems to have taken a few lessons in martial arts (most likely of the soft kind). Very good work.
Woah, this looks epic and so cool. There have been rumors about how much stronger Rarity is compared to Applejack, so this might ba a very tense battle, eventhough Applejack is the toughest pony around. "Strong" and "tough" are 2 very different things.