Right now most people use Port 6346. If this port if blocked for you, you won't be able to download from people using that port. However, you can use any port if you like...

Port 5190 or 8080 should be the standard as they are rarely used, but are used enough that they wouldn't be blocked. 5190 is the port AOL's Instant messenger, and ICQ uses.

Also, an ISP that blocks all incomming connections would extremely limit the functionality of any type of server, but that's very unlikely to happen.