At the beginning, my answer would be close to #2.
I was working in a university... Well, life was good... and I found myself being in love with this career, so I would like to improve myself from a good one to a great one... Being trained in a PhD program might be a good answer to me.
At present, after doing lots and lots of trials and errors in the past years, I found myself changing the answer somewhat... from #2 to #1....
The training within my gruesome PhD program has changed my mindsets though. From believing that being a good professor was "to able".... But, in my current opinion, it's not.... It is to learn the problems to its root, to plan elaborately how to tackle the confronting problems, and to execute the plans with available (and, in many circumstances, limited) resources.
As a father and a husband, being in the US taught me a more important lesson of what kind of people I am and what're things in life I do care about. I don't think that I could become the person I am today if I was in Thailand for all these years. Collectively, it has transformed me as a person, my perspectives, my dream and my knowledge about this world and its humanity.
Thus, from the place where I have stood as a human being, the most valuable of all is, in fact, #3. Above from chances to see the world, actually, it gave me time to reflect upon myself, my life, and life perspectives... It allowed me to contrast and compare between the two society in depth from culturul and historical points of views. I saw reasons why we are what we are and could darely predict what we are going to be.
And, as for #4, it's the least desirable of all, and it will definitely remain as it has been.. I will continue to wear my "Mr." as my lawful prefix.