GMO (genetically modified organisms) have a high destructive potential, so I'd say the best way to bring about the apocalypse is to create an organism that migrates far distances that is also resistant to ebola, so it can survive with it for a long time. My organism of choice would be a sea turtle or a duck, since they go pretty far. Ebola comes from monkeys, and monkeys can live with ebola for a while... So combine a monkey's ebola resistance with a duck and you'll kill off millions of humans in a matter of years.
What's your apocalypse?