Email is a bit tacky since they will have a lot of requests in the same mailbox. Also, you don't know which admin you're talking to, could be Googlefroggy or Klon, or maybe someone else like Kingraptor, or it could be the admin who banned you and will just delete the email...
That's why I vouch for PM in forums. I've seen quite a lot of PM in other forums as well, so why do we not have this implemented? It would be a great way to "chat" with other players, though I could use it for annoying deeds (Spamming "Lob" into someone's PM)