Ngwenya and Chavhanga actually ran the 200m for Zimbabwe when they were in high school (Chavhanga was 16!) After Ngwenya smoked Habana at the WC, i watched an interview with him on Transworld sport. They asked him if he thinks there would have been anyone who would have cought him in that same scenario. He said Tonderai Chavhanga would have cought him before he scored. He said Chavhanga is definately the fastest (because he always pipped him in the 200m races.)
Over the first 40m i reckon Shane's up there with the quickest, i mean the guy is sprinting before you know it - that's why he tears teams to pieces. But over the 100m he doesn't have the speed of some of the South African or New Zealand boys.