I have a dead Serebii account. I don't even remember what username or password I used, or even when I registered. I've also got an EGC account which I haven't signed into for God knows how long, as well as a Castelia City Forums account, which has also perished. If Neoseeker counts as a Pokemon forum, I've also got an account or two there, but again, both of them are deader than the dodo.
The two Pokemon forums where I'm not completely dead are PokeCommunity (duh) and Smogon, which is my main forum so to speak; I've been on Smogon since September 2011 and not about to leave anytime soon. I'm badged there, which means I have Community Member status and can do things most users can't. The one achievement that's given me that privilege is my frequently mentioned modship of their Pokemon Showdown server. I'm much more active there than I am here, although I'll probably step up my PokeCommunity activity at some point.