
According to some sources the
Black Fat Tail Scorpion is the most venomous scorpion, with highest
LD-50 (median lethal dose) about 0.16 - 0.5 mg/kg lethal dose. It has probably highest ratio of stings to fatalities in humans.

Number two is the
Deathstalker, often cited (Wikipedia) as world's most venomous or deadliest, that mean that he is may be the actual champion.
Also here are some more interesting species as the
Black Spitting Thicktail Scorpion which is highly venomous too and can spry its venom up to 1 m. The
Mozambique Hissing Scorpion can hiss. The
Yellow Fattail Scorpion despite of a little bit lower toxicity compared with the two species on the pictures, causes far more deaths every year, because it is far more common. The
Honduran Yellow-Brown Scorpion is the fastest species of scorpion.
However
there are different interpretations of what means "the most venomous", because the test animals are mice and the humans may have other LD-50 for some of the toxin.
Here we can see examples of the LD-50 in different animals for the same toxin.
Sources:
Centralpets,
Singnet, Wikipedia