The Justice League
Justace league has thausands of members compared to that of the little number from that of the Avengers.
Batman! He would have prepared the battlefield with traps and weaponry to take out each and every one of the Avengers. He would have probably sorted out plans to take them all out ages ago and probably got sick of waiting for a "maybe" so it was likely him that picked the fight in the first place! All of this plus his $$$, strategy and knowledge of his own teams powers would be enough for him to sit in his corporate box and watch... only he won't because he'll want a scrap! Batman could probably demolish all of the Avengers himself and it wouldn't be surprising if he has already built the exo-batsuit to do it. He had the Justice Buster on hand to take out the JL so it wouldn't be surprising if he had an Avengers Annihilator in the cave too. He could definitely take out Hulk or Thor or any of the Avengers 1-1 if he was in the HellBat Armour. The only one he would struggle with would be Iron Man. I reckon he could out-fight and out-strategize Tony Stark but I don't think he could out-build him. If there had to be other match ups and teamwork... Martian Manhunter is the most powerful telepath in the world and can scan every mind on earth in seconds and then reprogram an insane army of humans and metahumans to just overwelm them while the Justice League proceed to their poker night at the JL Watchtower. Aquaman could smoke them all himself if they had the balls to fight him in his backyard. I would love to see Wonder Woman take off her bracelets and go beserker on Hulk and then everyone else once she is done with him. Green Lantern would take out Iron Man because none of Tony's tech compares with constructs. I reckon the trash talk between Tony & Hal would be awesome! oh and then there is Superman, Flash, Cyborg, Shazam...
Dude, We have a Hulk!