- Argentinian -or-
All are acceptable.
I say (and write) "Argentinian" - I have no idea why. But blogs, and others things lately I have been reading, frequently use "Argentine" or "Argentinean". But all, in fact, are correct. It's a matter of preference, I suppose.
And for all you Spanish speakers, someone from Argentina is called an:
- argentino -or-
...depending on if you are talking about a male or female. Not capitalized in español. They have their own rules for capitalizing people, places, and things, sabes?
I know you were dying to know that. Glad I cleared that up, aren't you?