So surely the most effective disease would be highly contagious and lethal, but with a high incubation period of maybe a year? That way, everyone becomes infected then we all just start dropping.
...but yeah, you're right with one caveat: the success of a disease depends only on how far it spreads. The _most_ successful "diseases" are the multiple viruses who have completely integrated their genomes with the human genome, and so exist in > 7 billion copies on the earth.