whats your favorite kaijin

Not sure.

I'm not sure if the MonsterVerse has any kaijin, the closest they have are the ageless Iwi tribe and the always twin Chinese scientist ladies

I don't know if the Mothra priests and aliens exactly qualify.

The "-jin" suffix usually refers to "person" or "race" in the right context, which is why it's sort of weird calling the Shobijin, kaijin. And because there's no real English equivalent of the word "kaijin" it gets harder. The best English approximations have been "mutant", "supervillain" or just "monster". (Like kaiju.)

It seems aliens usually are provided their own "person" or "race" designation, ala, "X-Seijin" (literally the, "Planet X People" or "The People from Planet X") which has been retroactively dubbed "Xilliens" for international marketing purposes.

The "mutant" designation seems to fit closer to Matango, the H-Man, Showa Frankenstein, The Human Vaper, etc. Even the Titans from the Attack on Titan films have been called this. I don't think that works as well considering they're giant, but the debate comes from the fact they start as normal sized, ala Toho's Frankenstein.

well yeah but which one is your favorite?

Hmm... thats a good one. Do the Bilsaludo(post mechagodzilla absorbtion) count?

yes

futuriens

cool

Matango

You sir are a psychic lol