Choosing a Port for realtime multiplayer game

We built realtime multiplayer into a game we're developing. The game was built in Flex, the multiplayer server is based on Apache Mina. We're trying to decide on a port to run that server on that has the least chance of being blocked by company firewalls, school firewalls, etc. etc.

So far, we've been running the multiplayer server on port 443 assuming that no IT department would block the SSL port. But low and behold, we've found at least 1 law firm that blocks port 443 and it has me a little worried.

So, what's the least likely port to be blocked that we could run this multiplayer server on?

Has anybody had any particular experience with choosing a port poorly or wisely?