The one that gets its practioner killed! Oh, wait, do you mean dangerous to the OTHER guy?
It makes no sense to compare arts, because their uses are designed for different situations and environments. Their effectiveness will also vary with practioners' individual temperaments and talents. Unschooled streetfighting and warrior clan koryu both can be "deadly," but how can you "rate" their deadliness against each other?
Would you ask what kind of gun kills better than another? Whether a Luger is deadlier than a Glock? Both fire bullets, and if you get hit with one in the right spot, you're dead. All makes of gun fire projectiles at a velocity that is lethal. Their individual details may include variations in repeat-action, laser aim, or other bells and whistles, and some kill "messier" while others kill "neat" because of the kinds of bullets they take, but essentially all guns are deadly. You as the individual just choose the one that best suits your needs and interests. If you want to kill a lot of people within split seconds of each other, then you choose an automatic repeating weapon. If you just need to kill one or a few people on any given occasion, then a .38 revolver is fine. For point-blank to dispatch a bad date without him/her catching on until it's too late, a deringer is nice.
Deadliness aside, modern martial arts are exercises in theory, since we don't practice them to kill people (at least, I hope not). Ostesibly, we can use them to defend ourselves against attacks. But don't mistake the stuff folks do in the dojo with straight killing methods used strictly for dispatching -- for mercenary, espionage, or other professional reasons -- human beings to expedite a political or business objective. That's where the "deadly" resides. Even so, physical skills are rarely used for that line of work. Radioactive polonium slipped into a drink at a hotel bar was a much more intriguing killing method to remove an unwanted Russian spy (allegedly at the behest of Vladimir Putin) than would have been snapping his spine.
My $.02 anyway.