If I want to express my opinion about something, what's the most correct form? What are the differences? What is more formal and what more colloquial? For example, in Italian, nobody says In my opinion..., but My opinion is...
As FumbleFingers commented, none of I think … or In my opinion… or From my point of view… is as formal as In the opinion of the author. That aside, I think you have them listed in very-slightly-increasing order of formality.
More important than any differences in formality are differences in nuance and meaning. I think … may be seen as lighter-weight, more off-the-cuff, than In my opinion…, while From my point of view… may be seen as introducing a personal viewpoint or preference, possibly less objective than for either of the other phrases.
If you seek less formal expressions, consider It seems to me …, It appears that …, Apparently, …, Evidently, …, It sure looks like …. But of course for formal phrasing, rather than I think plus a conclusion, instead give a supporting argument or reason, followed by the word hence, followed by a conclusion.
"In the opinion of the author"?
This isn't formal writing. It's presumptuous writing. If a document needs a level of formality that avoids personal pronouns, then it probably also doesn't permit personal opinions. Personal pronouns are completely acceptable in formal writing. See any professional journal. Even doctoral dissertations allow personal pronouns.
Now, to your question. Each of these is acceptable, and each of these should be avoided.
I would avoid "I think" because it is a weak expression. It allows the reader to disregard your ideas completely. "In my opinion" is slightly better if you have sufficient credibility in the subject that your opinion matters.
All of these expressions tell the reader "I might be wrong, but here is my idea." All of them should be avoided as a form of hedging. See the article "In my opinion, I think that I believe this is bad writing." http://preciseedit.wordpress.com/2009/06/19/in-my-opinion-i-think-that-i-believe-this-is-bad-writing/
If you have a good reason for your idea, simply state your idea. If you're not sure about your idea, attribute it to someone else.