I'd say Twilight would win in a free for all between the 6. Although she has never shown any talent in offensive spells, TK and Teleportation can be murder. Next up is RD, she's insanely fast so she might be able to hit Twi before she has a chance to cast a spell, and then she can just pick off the rest. Rarity is next, because she can also use magic, but she is not anywhere near as powerful as Twi. AJ is 4th, she's strong and somewhat fast, and could take a lot of hits before going down. However she can't fly and isn't as fast as RD. I can't actually give a place to Flutters or Pinkie, because I have no idea what they could do, but Fluttershy has shown that she can easily beat a bear, and she could apply this in combat. Pinkie: No idea. Could take any of these positions, be stronger than Twi or weaker than Flutters when she's not in a combat situation.