Less likely:
Captain America - With the negative press of Steve Rogers being a Hydra spy, I'd say they would not touch this one.
Iron Man - I assume RDJ would be phased out one Phase 3 ends. Since he is the main character of the first MCU movie, I doubt he will be a Skrull.
Thor - I just don't see it.
Hulk - He's Hulk, plain and simple.
Dr. Strange - Too new of a charcater to pull a M. Night Shymalan type of switcheroo
Spider-Man - See above
More Likely:
Black Widow - After all, she had that "change your face mesh" in Winter Soldier. Maybe she is a shape-shifter after all.
Hawkeye - The guy uses a bow and arrow in the middle of a alien war and survives. Wouldn't doubt if he is a Skrull and thus why he has survived for so long.
Rhodie - How did he survive that fall in Civil War? Dude's an alien
Maria Hill - That's why she has been able to work up the ladder of SHIELD. She's an alien.
Drax - Sure, why not?