1. Attempting World Domination
Mr Fantastic - would be a simple question of getting hold of a Microsoft Surface. If Banner can trick-out an iPod, Reed with an overpriced table... lapto... err Surface, should conquer the world.
2.Being a mercenary/assassin
Spiderman - especially if we're talking real world, hardly anyone would be safe from him. Strength sufficient to take blast-doors off their hinges too, imo.
3.Fighting Petty Crime in your hometown (your powers can't get exposed)
Colosuss - not much call for fancy powers, just beat the shit out of whoever is in charge of the local gangs.
Spiderman would be overkill, imo.
4.Helping to better the world (However you see fit)
Reed - Bring on a proper, private space program.
1. Iron Man. Tony can solo the real world, tbh.
2. Iron Man. Nobody on the planet can stop you realistically. And hell, you can conceal your identity if need be, anyway.
3. Iron Man. Pretty sure a haxxed suit of armor armed to the teeth is going to put an end to petty street level crime once I repulsor ray someone's face off.
4. Iron Man. Didn't I mention how Tony could solo the real world?
This is between Animal Man and Aquaman (since you DO NOT get Reed or Tony's intellect along with their powers).
Animal Man for sheer versatility and power (***cough***Sun Eater ***cough***) or Aquaman for well roundedness (Super Strength, TP, water breathing, durability, etc...).
If I had to choose one and only one? Animal Man.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
You get Extremis with Iron Man's power set, incredible level of technopathy, and basically a "how-to" when it comes to your powers/suit. You won't be as smart as Tony, but you'll get pretty goddamn close if you want. You basically have instant access to any Earth-based technology available. And nothing we have in the real world comes even close to being able to take down pre-Extremis Iron Man, let alone Extremis or Bleeding Edge Iron Man.
If Animal Man's powerset extends to copying fictional animal powers from the DCU instead of being limited to real world animals, then, yeah, he should take this.
For Aquman and Animal man you only get real world animals that are proven to be real. So no Cthulu for aquaman and I know animal man has turned into alien creatures in the past.
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The Ankle Butcher, Patriotic Entity,
Earths Mightiest Hero, KURT PHUCKING ANGLE.
If you have even a "basic" how to with any of Iron Man's tech you basically have his intellect by default. Otherwise how would the suit have been created and maintained in the first place? Plus were would you get the initial funds to create that monstrosity? It must cost hundreds of millions, if not BILLIONS, of dollars in R&D. So you would need Tony's billions and his know how to even get started with the Iron Man armor.
Why would he be limited to only real world animals? It's not like Tony's tech is available in the real world either. It this thread was limited to just real world stuff, the Iron Man suit would be disqualified already.
EDIT
Well that sucks. So Animal Man isn't even a contender now. AM still has his best abilities anyway. So I change my answer : AM it is.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
You can have StarkTech and not be a super genius on par with Tony. And really, his powerset (ie. Extremis/his current suit) is an actual power that has, on panel, exponentially increased his cognitive ability in addition to several physical enhancements. I'm assuming if you get his powerset, all you get is the Extremis virus/armor, and not his funds, other tech, or resources.
Even so, earlier Iron Man suits such as the modular armor would be more than sufficient to cripple the world's economy and military forces. As the armor is portrayed now? Tony would most certainly rape the planet easily. And a shmuck would be able to do a shit load of damage. Any somewhat intelligent fellow could learn how the suit works thanks to the AI programming and Extremis tech and once they were marginally decent with the suit, could just start wrecking shit directly or subtly.