Paris green has an LD50 of 22 mg/kg, according to Wikipedia. That's one thousandth of the lethality of ricin, one fortieth of the lethality of sarin, half the lethality of sodium cyanide, one two-hundredth the lethality of alpha-amanitin, one fourth of the lethality of nicotine, roughly the same immediate lethality as hydrazine, methamphetamine, tetraethyllead, or aflatoxin B1, and only eight times as lethal as caffeine. It is very dangerous, and you should avoid getting it on you, but it seems unreasonable to describe it as one of the most toxic chemicals known. It is definitely not in the same category of nightmare chemicals as dimethylmercury, chlorine trifluoride, silane, or red fuming nitric acid. I'll admit that outgassing arsine if it gets moldy is pretty nasty, though.