Not sure about the scariest, but my favourite has to be the Anaconda in the Gold Reef City theme park in Johannesburg.
Great inverted roller coaster, with a double corkscrew and inverted zero-g loop
I probably enjoyed it more because I was in Johannesburg with work and was staying at the hotel in the theme park, but had 1/2 day spare before my return flight so dropped in and spent the morning there for about 80 Rand (£6)
http://www.rcdb.com/ig1169.htm
http://www.rcdb.com/ig1169.htm?picture=18
Great inverted roller coaster, with a double corkscrew and inverted zero-g loop

I probably enjoyed it more because I was in Johannesburg with work and was staying at the hotel in the theme park, but had 1/2 day spare before my return flight so dropped in and spent the morning there for about 80 Rand (£6)
http://www.rcdb.com/ig1169.htm
http://www.rcdb.com/ig1169.htm?picture=18