Match up wise, Iron Man will have an easier time in these degrees of difficulty (1 being hardest and 4 being least hard):
Magneto isn’t the hardest because Iron Man with good assists pretty much make rushdown Magneto dead. He can’t triangle jump you (much) and he won’t be able to dash in on you, therefore cutting off his attack options completely and just running away until he can DHC/Counter in someone else who can probably fight him.
Sentinel is 3rd because of Iron Man’s ability to catch him in the air sometimes with air combos (IE, Sentinel sjs -> immediate fly, Iron Man can do quick air combo, possibly into UnFly combo) and because Sentinel without a good AA is sort of weakened.
Cable ranked 2nd is realistic. Iron Man will lose this fight 2/3. No matter what assist Cable has, Iron Man cannot risk a lot of things in order to attack and must rely on either catching him with an infinite setup or hopefully being able to DHC/Counter out Iron Man for someone better. Otherwise, a lot of Iron Man’s normal attack options are shot to shit because they’re all AHVB vulnerable.
Storm is 1st because 1. She runs away no problem from Iron Man. 2. She has the best offense against him if she has Sentinel backing her up. Fighting Storm is pretty much crap, cause if you try to tri-jump in, you’ll get s.RH and you’re dead. Turtling is pretty much useless and you’ll just be running the clock down in HER favor.
Iron Man does have his occasions when he’ll be able to win a match up against any of those 4 if/when he’s able to do the infinite, but if you’re trying to do anything else, I doubt it’d be a big impact. Otherwise, stick to using big 4 vs big 4 since that’s the only real way you’re gonna win.