Sin, in itself, has a religious implication and conotation, so Im just going to have to go against the grain here and offer up my best scientific opinion.
In interpreting and/or conceiving the ultimate sin, one must first address the word ultimate. When I think of "ultimate" , I think of terms like grand scale, volume, abomination, armageddon, mass extermination......to name a few. Hence, I would have to say that mass genocide to the most extreme degree of eradicating all of mankind would be one version of an "ultimate sin".
An example of this could possibly entail concocting an elaborate plot to devise not only a deadly biological element with which to eradicate a species, but also a means to successfully distribute said element. As to the realistic possibility of this ever occurring,..well..isn't the whole point of this to both exercise your opinion , as well as your creative intellect? I think so.
Conversely, one could interpret this scenario as anything but a sin, in the interest of the biological preservation of the planet. Again, this delves into religion and scripture, so a true answer to this query would require some sort of biblical reference. Anyway, slaughtering everything on the planet with an opposable thumb has my vote, with suicide a close second place.
