A browser that can send electric shocks down the internet to people who post stupid threads, in which they abuse the english language.
The correct answer to your question is "I would have a superhero's intelligence". Obviously 😛
That and maybe, if I had to choose one of the great minds from Marvel, I'd choose Banner or maybe Doom.
Banner would basically provide everything Stark can, plus a far greater degree of self control, willpower, psychological understanding (ability to manipulate the other geniuses) and even greater scientific intelligence combined with far greater technological expertise.
With it, I'd figure out a way to harness fundamental forces like magnetism and radiation to produce phenomena like levitation, "over unity" energy production, space and time travel ...
That and get myself an island and a tonne of droids to defend it against anyone and everyone.
Doom would be my second choice, as without the magiks he's less than himself.