Here is my take of which character best fits each class.
Artificer: Iron Man
Bard: Peter Quill/Star Lord
Paladin: Captain America
Rogue: Black Widow
Sorcerer: Captain Marvel
Warlock: Scarlet Witch
Wizard: Dr. Strange
Those are good picks. Obviously, the MCU characters usually have a narrower set of abilities than D&D character, but from a « take a class that gives you the abilities you want to recreate » stand point, as opposed to a « take a class whose core definition is closest of the character », it works.
Captain America is a good fit for Paladin, both in righteousness and abilities, capable of the ocasional « smite » and leader auras. Many Paladin spells can emulate the shield. And clearly not an OD&D one, he associates with a thief(rogue)!
iron man as artificer is a no brainer. So is dr. Strange as wizard. Quill as bard is a pleasant epiphany. Dance-off! Hulk barbarian, well yeah. Black widow as a rogue is quintessential.
i like Thor as cleric, but more the self buffs type, not the healer. He’d make a remarkable 3e cleric... Black Panther is a monk. Spider-Man, more on that later.
Ant-man. Changes size, ‘summons’ ants, speaks to them... yeah, Druid works. With criminal background.
I’m not sure to what extent Hawk-Eye is a Ranger... could be a fighter too. But he’s got specialized arrowheads, seems resourceful. Ok, hunter ranger it is. Gamora is more fighter-y at any case.
Scarlet Witch seems more limited than Marvel, and relies more on at-will attacks. That’s a warlock in action. Marvel gets sorcerer.
thats for classes, but leaves poor Spider-Man out. Well, he’s in and out of MCU anyway... So, Spider-Man has an iconic danger-sense ability, shoots web (as spell), and swing on web (short-burst flying ability?) and does acrobatics. He’s hard to case as a D&D character. So lets make it the archetypical multi class. Dange-sense is barbarian. Actually, he’s the elusive Dex-barbarian build (eagle totem). Then he multi classes as sorcerer for spider walk and web. So he can « rage » to soak damage and resist doc octopus’ grapple better, but then he’s grounded (can’t cast spell). Weird build, but he’s a bit of an oddball character himself.