WHICH AVENGER IS SECRETLY A SKRULL PLOTTING TO TAKE OVER THE EARTH?

I think in terms of which character would make the most sense to be a Skrull and would add to one of the biggest twists in the MCU would be Tony Stark after he went into the wormhole at the end of the first Avengers movie. Ever since that movie, he's a been a changed man that's done more to destroy the Avengers than anyone; destroyed nearly all of his suits, created Ultron (which he claims to have done by 'accident'), pushed forward the Sokovia Accords which if anyone knew Captain America knew he would inevitably oppose and tried to kill Captain America's best friend. Its highly unlikely that Robert Downey Jr. would be onboard to allow his character to be retconned as an evil alien, but in terms of storytelling it would be perfect, and even allow fans a return of the old pre-Avengers Tony Stark.

But in terms of realistic expectations, I don't think any of the original founding Avengers will be revealed to be a Skrull, not even Hawkeye;

- Captain America is definitely dying in Infinity War,
- Robert Downey Jr. wouldn't agree to making Tony a Skrull
- Thor has done too much to help the Avengers for him to be revealed as a Skrull and is also a fan favourite hero
- Black Widow is one of the biggest fan-favourites of the founding lineup and also one of the few originals who might return for more movies after Infinity War Part 2. No way she'll be turned into a Skrull.
- Hawkeye has neither done anything to help nor hinder the Avengers. Doesn't make much sense for him to have been a Skrull all along and not done anything to weaken the Avengers or improve his own standing on Earth.
- Hulk is too much of a fan-favourite to be revealed as a Skrull and I don't think Mark Ruffalo will be returning anyway after Infinity War Part 2.

I think in order to not upset non-comicbook fans of the movies, none of the main MCU heroes (The founding Avengers, the Guardians of the Galaxy, Ant Man, Black Panther, Spider Man, Doctor Strange and Captain Marvel) will be revealed as Skrulls. Keeping in mind logically which characters have done more to harm the Avengers, SHIELD and the rest of Earth's defences in general, and who's reveal as an evil alien would be major enough to cause a twist but wouldn't upset too many people, I'm going to predict the following;

- General Thunderbolt Ross - This guy is the likeliest candidate and makes the most sense. He's been around since the beginning of the MCU and that, along with his age, means he could easily appear in a flashback in Captain Marvel showing him being replaced by a Skrull. He's done a great deal to weaken the Avengers, by going after Hulk in his solo movie with Edward Norton and introducing the Sokovia Accords, and all that along with his political standing on Earth as the first Secretary of State makes him an excellent candidate for a Skrull double-agent with a lot of power at his hands to ease the Skrull invasion.

- Sharon Carter - As @noahthegrand suggested, Sharon Carter is a prime candidate to be revealed as a Skrull given how close she's made herself to Steve Rogers and the rest of the Avengers, helped aid Rogers in taking down SHIELD/HYDRA who could've stopped a future Skrull invasion and it could easily well be interpreted that she manipulated Steve into opposing the Sokovia Accords, driving a wedge in the Avengers. Practically as well, Emily VanCamp is probably going to be around for a long time after Infinity War, given that she's only appeared so far in just two MCU movies, and once Rogers bites the dust in Infinity War, there's very little that her character could do without Steve unless she's taken in a different direction. Revealing her as a Skrull who had been manipulating Steve and the Avengers all along would be an excellent twist.

- Hank Pym - Another potential candidate who fills up a lot of the criteria. He had a strong connection to SHIELD back in the 90s when Captain Marvel will be set in, has a connection to the current Avengers (somewhat) and his character was revealed as a Skrull in the original storyline. Revealing Pym as a Skrull would make a lot of sense, pay tribute to the original comicbook storyline and could also pave the way for greater development for Scott Lang's character, who'll inevitably be more of a major character after Infinity War when most of the founding Avengers depart the franchise.

- James 'Rhodey' Rhodes - Rhodes is another character who's been around since the very beginning of the MCU and his reveal as a Skrull would make for a brilliant and emotionally devastating twist. He had clashed with Tony in Iron Man 2 and stole one of his suits to seemingly hand over to the US government, weakening Tony and improving Rhodey's own standing with the US government which would be very beneficial for a secret Skrull wanting to aid his allies in weakening Earth's mightiest heroes. Rhodes has continued to conflict with other characters throughout the MCU, most notably in Civil War when he backed the Sokovia Accords and played a personal role in arresting Steve, Bucky and Black Panther, seemingly handing them over to the US authorities for good and allowing them to rot in prison (perhaps not Black Panther with his diplomatic immunity but you get the idea). But at the end of Civil War, Rhodes was paralysed from the waist down as a result of an explosion caused by Vision...Or was he? Disguising itself as a paralysed man close to Earth's mightiest heroes would make for a great disguise for a Skrull, who could take advantage of his 'allies' letting their guards down around him and then eliminate them when he could.

Imo those are the four best candidates for being secret Skrulls, along with Iron Man (But RDJ will never agree to it), but if the Secret Invasion storyline is truly done in the MCU, I'm dubious as to how many major characters will be retconned as Skrulls as I'm not sure Marvel Studios will have the guts to go through with such a massive decision to retcon some of its major characters as Skrulls. Back in Winter Soldier, only a handful of minor SHIELD agents and Senator Stern were revealed as Hydra agents, and they weren't major characters in the grand scheme of things. Realistically, I can only see General Ross and maybe Hank Pym being revealed as secret Skrulls and that's about it unfortunately.

I do like the idea of Sharon Carter, makes sense as well for the reasons you wrote.
Would love a Hawkeye and Natasha team up movie.

I really don't want any of the Avengers to be skrulls, I'd hate it tbh, after so many years it would be hard to swallow that one of them is an impostor. And I don't want Sharon to be one either, even though it would kinda make sense, but she's been so underutilized and then she has gotten so much hate that this would feel like a big f*ck you to the people who like her imo.
My pick is Everett Ross, or one of the Avengers, but after infinity war. I like the idea of doing something like secret empire, but in this case instead of a hydra take over we'd get an alien invasion, and no one would suspect Steve. That'd be cool.