These are my favourites at the moment. I won't spell out the names(!) but you can see for yourself. Given the price of postage it's worth buying a plastic 1 litre bottle rather than several small glass bottles.
Having said that, Tabasco Habanero sauce is fantastic. It is not only as hot as hell, it also has flavour, as it has various West Indian ingredients. In comparison, the normal Tabasco just tastes salty and weak.
Haha try the habanero version and you'll certainly get more than one serving per bottle! And as I say, it actually tastes nice as well as being dangerously hot.