Okay, I picked Applejack. Here's my reasoning.
A lot of people might want to pick TS because she has Alicorn magic, however she also has pretty good composure and I can't see her becoming violent unless she was up against the BBEG (like Tirek), especially since she's the type of pony to give people second chances and becomes more naggy than anything when she's angry. (Seriously, if she didn't get violent after "three's a crowd", she deserves an award for composure.)
I didn't choose Fluttershy because, although she can become quite angry and violent, this doesn't change the fact that she is physically rather weak. Also, Power Ponies strength doesn't count!
I didn't choose Pinkie Pie because her anger doesn't turn into violence--it turns into depression. (Party of One) Also, her anger/strength/speed is usually only exaggerated for comedic affect in light-hearted moments--I doubt she's really that aggressive in serious circumstances. Otherwise, she could have wrecked Tirek.
Rarity isn't the type to become violent either. The most we've seen her anger do is send people on nasty guilt trips with a swift tongue-lashing. Well, i suppose that is dangerous in its own right...
Rainbow Dash...well. She's fast, and stubborn, and loyal, and is a hothead, but she's also not terribly strong or terribly tactful. At least, not compared to our winner...
Applejack. Strong. Fast. Stubborn. Prepared. Willing to be the "bad guy" for the greater good (a side-effect of honesty). Not only is she physically capable of violence, but she's mentally capable if you upset the balance. I think "Somepony to Watch Over Me" proves that.
But if we're talking raw strength and not just attitude, It'd probably be Twilight. Duh.