Usually examples of mutualistic relationships don't involve one "coming first", since they co-evolved...They evolved together. But if we want to be technical, I would say that the flower came first because there are other mechanisms of pollination. Most gymnosperms, for example, are pollinated by wind. Also, in other earlier plants, like ferns and bryophytes, pollination through the use of water carrying the sperm. You might not call these "flowers", but they are basically proto-flowers that came first, which allowed opportunity for insect pollination to begin, and lead to the bright and colorful flowers that we enjoy today.
