For my list, I'm going to attempt to avoid the classic "God Cards" (aka Goku, Superman, Hulk, ) and, as an added challenge, try to keep the list as human as possible, while still being badass. So while these people may be kicked out of the arena by some other characters that exist, that doesn't mean you should underestimate them for a second.
Team Leader: Artemis Fowl
One of the greatest strategists in fiction, Artemis Fowl has a particular knack for taking on larger than life foes. With his only powers being his own incredible intellect and being comically rich, there really wasn't another option for a leader. He understands how to play to the strengths and weaknesses of whatever team he's given, and can analyze an enemy quickly and masterfully. While having no impressive fighting skills himself, this doesn't make him any less of a threat.
1st Round Pick: Tony Stark
Yes, yet another genius rich guy. But I would like to note that while he IS the Iron Man, that is not the intrigue I have with him. It's his genius and adaptability that makes me want him. Not the suit, but the man inside it. He works until the jobs done, and despite his pompous nature, he knows how to work in a team (to an extent), and will be crucial in getting the team the best gear imaginable. One of the greatest skills humans have is their ingenuity, and he's the perfect expression of this.
2nd Round Pick: Punisher
A combat mastermind, trained in every discipline imaginable. Adaptable and resilient, he's a one man army. When combined with the strategic advising of Fowl and the equipment of Stark, I can only imagine the things this man could achieve. he can operate as an assassin or a soldier as necessary. This is a man you do not want fighting against you. Especially if you don't know where he is.
3rd Round Pick: Deathstroke
This is the least human of the bunch, with a healing factor, super strength, super speed, and a whole bunch of other stuff that starts with super. Also, unlike the others on this list who are either heroes or antiheroes, he is usually classified as a villain. But with his strategic mind, years of experience dealing with superheroes, villains, and aliens alike, and incredible combat skills, he is a force to be reckoned with. Also, his experience as a villain gives a look into the mind of such a person, and a more cold view of any situation. Despite how cruel his suggestions may be, sometimes it just may be the final option. Whether it's close range, long range, stealth, or assault, Deathstroke can handle it. Working in combination with someone like Punisher would be a sight that I personally would pay to see.
There it is, my dream team. They compliment each other, have a fear factor a mile wide, plenty of resources, and dammit they'd look cool together. For the most part, they are human, and some may consider this their primary downfall. Alongside this, some people (Lookin' at you, Deathstroke) on this list don't exactly concern themselves with civilian casualty. This is an issue, but I believe the rest of the team could keep him in check. I am confident that these people together, with their combined wits and combat experience, would be able to tackle any foe, given a little bit of prep time and access to the nigh infinite funds of the Fowl and Stark family bank accounts. This, obviously, is just an opinion, however, so feel free to debate it.