Content campaign and signature are the most profittable but it s depend on the bounty it self and how much they allocate to each one
For me i participate in twitter , FB , and article ,thats what i can do for now but i will try to learn much to participate in others
good starting , while you cant participate in signature look for other options that will give you good rewards. I recommend you to join in translation bounty cause that bounty has the biggest reward for good translators.