The definition of a supercar is one that has to constantly change as technology changes the playing field; a hatchback now can be faster than the Ferraris of not too long ago. Again, the quantities play a part, for example a 911 is a fantastically fast car, but there are so many of them built that it's hard to position where it sits. Likewise obviously a Ferrari 458 Italia is a supercar, but is the older F430? What about the previous 360 Modena? These cars start their lives as supercars, but over time fall out of the fray leaving it to a newer niche.
My view on the definition of a supercar covers cars from Audi R8 V10, Porsche 911 Turbo and Aston Martin V12 Vantage up; the small quantity, faster versions of their respective models. Any current model Ferrari, Lamborghini, McLaren sits within this fold too.
As more and more cars reach the supercar fray, where before you would have said the Ferrari Enzo, Porsche Carrera GT and so on would be supercars a decade ago, there are now so many cars with performance in reach of these that the newer term 'hypercar' has naturally been created to encompass the latest models; Bugatti Veyron, LaFerrari, McLaren P1, Koenigsegg Agera, Pagani Huayra and all. We're now in a world where the Koenigsegg One:1 that has recently been announced is being position as a 'megacar' to create a new league above these others.
The term is so varied that different people will always use it in different ways, and albeit some will overuse it, I don't see that as a problem. For example, for somebody who rarely sees a special car, a Ferrari F355 could be incredible and to them quite rare. Whereas for someone based in central London for example, you could see a constant stream of 458 Italias, which barely catch your glance any more.
Whether I personally see a car as a supercar as I say can differ vastly from someone else. To me an Audi R8 V8 isn't particularly rare, nor special, any more and equally the price has now reached reasonable levels for purchasers. It certainly looks fantastic, but the performance is lagging behind, whereas there would be many people who would class it as a supercar immediately, which is equally fine. Location and involvement in the automotive world plays a big aspect in this, because at the end of the day, it's down to how rarely you personally see a car of that class.
Regarding the 'worst supercars', I'd argue that's a title that can't really exist! Every car is built as the dream of the founder or designer, and ultimately it's down to a customer to choose to buy it or not. If there are few purchasers then the car becomes rarer and in turn that makes it more special. Take the McLaren F1, now considered one of the very greatest cars out there, but in it's day only 64 road-going models were ever made due to lack of demand.