I think "deadliest" is extremely subjective. I learn sword techniquess in karate.
I could cut off your head with a samurai or puncture your lungs with kamas...
I also could kick you in the head hard enough to knock you out for minutes, and then proceed to strangle you while you're out cold. An easy kill...
I could punch you in the throat and you would choke within a couple of minutes. A single strike to your death...
See, I could list several very real world practical things from just one art that would kill your opponent with one strike.
Fighting for sport with a particular art is one thing. Applying it to the extent of death upon another is something else.
If you're just going for the K.O or T.K.O, then several different arts, when performed at their highest level, are nearly equal anyways, in striking I mean.
In muay thai, it seems to me that it is a highly offensive art. Kicking or striking something as hard as you can and kill the other guy before he kills you while taking blows, but getting him first.
I am not big on the knowledge of muay thai, but is self defense (I mean practical stuff) even taught? For example, if a guy bear hugs you from behind and locks your arms down, a roundhouse or an elbow is impossible to throw. So, how do you get out? Do you learn this kind of stuff, or do you just kick the shit out of banana trees until your legs are without sense?
Jiu jitsu is very practical if someone jumps your from behind and takes you down in a street fight. Snapping an elbow, wrist or ankle, blowing out a shoulder or knee, or a nasty choke are all very effective ways to dismantle your opponent.
In sport, it is equally as effective. But, if you are a BJJ guy that can't hit worth shit, and you can't seem to take the guy down, you're gonna get fucked up.
They all have pros and cons, and any art can easily kill an opponent if you know what you are doing.
To me, saying "muay thai is the best" or "Tae Kwan Do sucks" etc is really an uneducated thing to say.
Now, someone who knows a vast amount in several of these arts, and has mastered the skills, will certainly be a more difficult opponent and that person will also know that there are things to be taken from all arts to be formulated into the most optimum combination of skills for defending oneself or to just kick some ass if someone is talking shit to your family or something.