As far as i'm concerned, i think the most fulfilling career is dependent of each and every person. I love my current job, it's challenging and it's not something in which you find yourself doing the same thing over and over again. So that's me, i find challenging environment to be the most fulfilling.
Another person might prefer creativity over challenge. Coding all day might look fulfilling to another person...etc.
I would suggest for you to understand what you really want to do with your life, the field that interest you the most, and go for it. All in !!
Hope this helps :)