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