'None of the above'
I'd have no time to actually programm/work/sleep/eat/etc. if I 'did' usenet or irc regularly, much too much going on there. I looked at use perl and couldnt figure out what it was for. Slashdot is way too busy, and much too difficult to find the actual information in all the answers to articles. Never been to LiveJournals (?) or JavaJunkies, don't use AIM (I assume you mean the AOL Instant Messenger by that, you never know ,)

My vote would have to be the Muds I hang out in usually. (Or these days just starting my Perl script that pretends to be a mud and sends/receives the intermud chat-channels.. :)

The big problem with most on-line communities (including this one) is, that you either have to hang out there more or less 10hours a day (or more, or log everything) to keep up, or you don't really belong. I've been and gone at quite a few muds, and am amazed at how many people just seem to ignore you, if you go away for a while (upwards of a quarter-year) and then come back.. (But you asked about favourite, not most-frequented, so thats irrelevant I guess :)

C. *mudding is the best!*