For me it's the big three:
Carlton, for denying us the '72 and '82 cups, and their years of salary cap rorting and tanking. I'm enjoying their current plight SO MUCH.
Collingwood, for their role in our demise all those years ago via the draft wars. Travis Cloke is a constant reminder of this treachery.
Essendon, whom I used to be fairly indifferent about until the "supplements" saga, and their constant playing of the victim.
But I also hate Sydney, for the constant leg-up the league has given them over the years. Often at our expense (see Tony Lockett.)
GWS and Cold Coast.
These two soulless franchises manufactured solely for the AFL's greed in conquering rugby territory, were allowed to feast at the draft table, while we picked up discarded chicken bones when trying to re-build.
North Melbourne. A nothing club forever sucking at the AFL's tit. If they'd had the good grace to go to the Sunshine State we wouldn't have Cold Ghost.
Finally, I can't believe how many of you let Geelong off the hook. I HATE THEM!!!! They've owned us for years - ever since the days of Gary Ablett senior. Then, when we finally get rid of him, his bastard son steps up to continue the torture. Then they steal Ottens off us, and unleash him against us at the Dome in 2007 in a hundred and fifty point smacking. And we haven't beaten them since. I'm still waiting patiently for our revenge on that one. When the day comes it will be very, very sweet, and I will personally walk around whatever stadium the feat is achieved at, and let every Geelong supporter know all about it.