Comic Book Combat, Fight #5: Synthezoid v. Martian
Ok, here's another no-brainer, at least as far as power sets goes: Vision v. Martian Manhunter.
Both of these characters have similar powers: superhuman strength, speed, agility, density control, flight (although in Vision's case it's less flight than it is floating on air while near-intangible...), and vision/energy projection powers. The powers that Manhunter has that Vision doesn't are his shape-shifting ability and telepathy.
J'onn J'onzz, the Martian Manhunter, is the last survivor of his people. A law enforcer on Mars, he was ripped through time & space via a freak accident, and deposited on Earth sometime in the 1950s. Between the time he left his home and the time he arrived on Earth, the entirety of his people died. He spent the next few decades on Earth as police detective John Jones.
The Vision's history is somewhat convoluted and unconfirmed. He is either the repurposed body of the original Human Torch, an android from the World War II era, a duplicate android built using the Torch's spare parts, or a completely new being unconnected to the Torch in any way. No one knows for sure.
Now with that bit of background, on to the fight...:
The fight, as I see it, could hinge on one thing: J'onn's weakness against fire.
The Martian aversion to fire is psychological as well as physiological and is linked to their shape-changing abilities. There's a Martian legend of an apocalypse born in fire that, if lit, would burn through the Martian people, mind-to-mind, killing all it touched. That is, ironically enough, exactly what happened not long after J'onn was ripped from his planet & people. Thus, Martians are taught to respect & fear fire, lest it take root in their minds & burn them inside and out.
Vision, through his Avengers & SHIELD contact, would probably know of this and use it to his advantage.
Conversely, J'onn's shape-changing ability would likely be a short-lived advantage. If J'onn were able to infiltrate close enough to Vision undetected, he could land a blow or two before Vision were able to recover, provided he's still functional. Remember, J'onn is a Superman-level force in both strength and speed. If the option of incapacitating Vision were the only one available, I'm sure J'onn would have no compunction in using all the strength at his disposal to stop Vision, who is, after all, still just a machine.
I'm sure, however, that once the fireworks started, metaphorically speaking, the actual fires would start in short order as Vision tried to detect J'onn in whatever guise he may use to get close enough to him to waylay him again.
J'onn's only chance is to take the fight to a place where starting and maintaining a fire would be dangerous or otherwise impractical or impossible, like a desert, ocean or ice patch.
Doing this also makes his shape-shifting for stealth more difficult; fewer people or other creatures amongst which to hide. Although he could still render himself invisible or change into a more combat-ready form.
I'm not sure how their density-altering abilities would make either combatant harder to hit. If their strength is relative no matter their current density, altering their density in that way would be futile.
So, you remove Vision's advantage; J'onn's weakness against fire, and you remove J'onn's advantage; his telepathy, and you functionally negate either's ability to escape harm by altering their densities, and you're left with J'onn having a marked advantage in his shape-changing & invisibility. Both could be used to great effect against Vision. It's unclear to me how Vision could counter J'onn's abilities (with the possible exception of his invisibility, because he still generates body heat even while invisible).
I've gotta give the win to Martian Manhunter here. All of Vision's advantages can be blunted by choice of battleground or equivalent powers. J'onn, on the other hand, still has powers Vision doesn't. If J'onn uses them effectively, he could easily put Vision down for the count.