Originally Posted by The Question
Well, I'm assuming we're going for the most popular/iconic line ups. Those being:
Now, the major clicnher in this fight would really be The Flash. Thor, while not having Superman's reflexes/reaction time, has a great deal more range, seeing as how he can conjur up a pretty nasty storm, ao thoaw two are pretty evenly matched. Captain America would defeat Batman, Scarlet Witch could hold her own againts Green Lantern, The Vision would be able to deal with Martian Manhunter, and Giant Man is capable of reaching sizes where he would be a formidable oponent for Wonder Woman and Aquaman. The Flash, however, trumps Quicksilver in terms of speed, and could conceivably disable the other Avengers at high speed, or at least mess with them.
Also, you have to consider tactics. Thor and Scarlet Witch would most likely pool their powers to teleport both groups to some secluded location, possible a place where the Avengers would have the upper hand. Also, Martian Manhunter would have the League in a telepathic link, giving them a greater stratigic advantage. Also, Vision (and martian Manhunter, who has similar powers) could preform a "Superman Smasher", as I call it. Basically, he would fly into the upper atmosphere, increase his body's density to maximum (where he weighs roughly 90 tons), and then free falls, slamming into his desired target (most likely the oposition's heavyest hitter), slamming into him or her at terminal velocity. If either Vision or Martian Manhunter did this, they could definately take out the bigger players like Thor and Superman quite early in the game.
Really, depending on how well they stratigise, it could go either way.