Summarize this article:

You can pick any flower that you want for your boutonniere. Make sure the flower has a sturdy stem so your boutonniere doesn’t wilt.  Pick a flower that’s the same color or a complementary color of your clothes. For example, if you were wearing a purple tie, choose a purple or yellow flower for your boutonniere. Use artificial flowers if you don't have access to fresh ones.

Summary:
Choose a large flower for the middle of your boutonniere.