Or Graham Coxon’s Song for the Sick
This. You may be thinking, “Don’t let the door hit you on the way out,” but no need to put it, or any related sentiment, into spoken words.
He’s leaving; any negative statements you add won’t gain you a thing. So leave well enough alone.
I guess, as mentioned, it depends on why you hate the person. If it’s just office stuff, remember that they will be out of your life and there’s no reason to hate them anymore. I would be an adult and just say “Hey, good luck on the new gig. I mean that.” The hard part is to actually mean it. Don’t forget that being nice can also be a pretty cool weapon. 
Now, if the person is a truly disgusting human being who beats his wife, kicks puppies, or puts ketchup on hot dogs, then I would do nothing. Why go there?
When people I despise leave the job, I just ignore them. If they insist on saying goodbye, I shake their hand and say goodbye. If they don’t come around to me, I don’t make an effort to come to them. There’s no need to be rude and you must at all times be civil.
It would be unprofessional to make more of it than good bye or good luck. As people have said you could end up working for/with this person again, plus your co-workers and bosses will be watching and noting. You’re better off saying nothing if you think you won’t be able to refrain from making a snarky comment.
“Remember that lunch you packed that tasted somehow off? Heheheh”
I think this is the best advice. I like the part about acknowledging what a good job they did with the transition. You don’t need to like someone to say they did a good job when they did a good job.
Or, you can try: “Hope you’ve got the whole ‘would you like fries with that’ routine down, because you’re gonna need it”! 
I do not recommend the linked approach, but when I saw it, I thought of this thread.
You could always organize the going-away party, and be waaaaay too enthusiastic about it.
I like the part about hiring a band.
Fantastic. I can just see each person trying to decide who was who on the list.
If the person is going around saying goodbye, I suspect they may avoid you during the process.
If they do come to you, just say “Good luck.” Why should it be more? You aren’t expected to deliver a State of the Union address. The guy is leaving. You are staying. Why say more than that, even for a departing friend???
BTW, update us, please.
Just wave at them and say “goodbye” and be done with it, if they expect you to say anything. Don’t waste your breath saying “good luck on your new job” or “I know we hated it each other”.
You may well encounter this person again, if it’s a small field. You might even end up working under them.
[Arnold Rimmer] Look, I’m not much good at big speeches, and I know I haven’t always been an easy guy to get on with, and I know, that given the choice, I wouldn’t have chosen you as friends, but I just want to say, that over the years, I have come to regard you as people I met. [/Arnold Rimmer]
Just say, good luck out there.
We’re all counting on you.
ETA: Seriously, WTF?
You’re lucky enough to have a disliked coworker leave? Any coworkers that I don’t like have a way of sticking around until the bitter end.
Seems like the perfect opportunity to demonstrate your professionalism by saying something at least blandly positive (such as ‘all the best for the future’). Whilst it might be fun to imagine tearing the disliked person to shreds, that will only reflect badly on you, in this specific context.
Might be worth pointing out that this is a two-year old thread so I suspect the OP no longer requires advice.
D’oh