Is it going to be pronounced like "ray gun" or like "ree gun" or like "reeg Ann?" If like ray gun, then Rhaegan will get you there for the pronunciation (in other words, teachers probably won't be pronouncing it wrong from reading it on a list and embarrassing the child), or Raegan would do too. A bonus of Raegan is that the nickname Rae could be used, which is a very pretty name.
Rhaegan is my favorite spelling of the name.