The Aang i'm talking about here, is when he is at the peak of his Avatar abilities, so when he is a man, not when he is still a kid mastering the elements. That being said, his powers are the use of the four elements. These include earth, water, fire and air. He can bend the elements to do pretty much anything! Since each of the elements are patterned after a type of martial art, it would make sense to say he is extremely skilled in many kinds of martial arts. Anyways, let the challengers approach!
If you've played any of the recent Super Smash Bros games, you will probably recognize Ike here. I have never played the original games he appeared in, (Fire Emblem) so I will be talking about his powers from the Super Smash Bros games. First off, he is very strong. His moves do a lot of damage. He is a master swordsman, as well as having the power of Great Aether, which is basically using a magical flaming sword and hitting people a bunch of times with it. His weakness though, is definitely his speed. He is quite slow, so that is a big reason he would need help fighting Aang.
Now every team needs a leader, and the brains to plan a good attack. Of course Batman would be a much better choice for planning and for everything really, but I figured Batman could take Aang down by himself, so he would overpower this list. Anyways, Nightwing was trained by Batman in martial arts, tactician skills, he was the leader of the Teen Titans, and he has his own city to take care of. He could get the team together and also land a few hits on Aang with acrobatic style!
3. Jean Grey
Okay first of all, I am not talking about when she is hosting the Phoenix force, because she could easily take on Aang alone with that. I'm talking about when she's just regular Jean Grey. With her amazing telepathy and telekinetic powers, she would prove to be very detrimental to Aang. Now, the reason she couldn't just turn Aang's brain to mush, is because Aang is very in-tune with his spiritual and mental side. He is very strong mentally, and he could use some of his spiritual powers against her. She would still be able to hurt him with her telekinetic powers, and her mental attacks would distract him from being able to fight his best.
I figured I needed a character that's actually on the show with Aang. Now you're probably thinking she would never fight Aang because well, she's his wife! Normally that would be true, but I'm saying that Jean Grey was able to do a little mind control on her to help them fight Aang. I would think she was someone who isn't easily brainwashed, so it would take someone as powerful as Jean Grey to do it. Anyways, she is a master water-bender, with the ability to blood bend, (basically control someone) and ice-bend. She basically taught Aang water-bending, so she ought to be dang good at it!
Anyways, those are my picks for 4 people that could bring Aang the Avatar to his knees!