I'd honestly like to see the Mole Man be the first villain they face. But out of these three, Kl'rt needs the most exposure. So I'll go with Super-Skrull.

Super Skrull would probably be an ideal starting antagonist as those perfidious green-skins are not only established but this could also serve to showcase their capabilities and set the stage for the upcoming Secret Invasion MCU event.

Doctor Doom could be present but in the background using his newfound powers to restore his beloved Latveria or beginning preparations thereof such as gathering resources and magical knowledge. Perhaps there could be a spin off film similar to the upcoming Black Adam.

Out of these, Doom. I think Annihilus & Mole Man should also be options though

Super-Skrull

Doctor Doom.

I feel none of them, I guess that since phase 4 is related to Kang much, therefore RAMA TUT is the one in my thought

Definitely Doctor Doom