The traveller’s guide to Mexico makes it all clear!
“The Scoville scale measures their (chili peppers) hotness in heat units (SHUs). Cayenne, the chili Tabasco sauce is made from, ranks a respectable 3rd place behind the hottest chili known to man, the habanero. The green Bell pepper is a 0! See how your favorites stack up.”
De Arbol 15,000-30,000
..just a correction, the hottest chilli according to the Guinness Book of World Records is ‘Bhut Jolokia’ with an SHU of over 1 Million. Habanero comes second;)