The Texas Instruments Nspire is the best for plotting graphs. It's a high resolution display so things look very, very clear whereas on some of the cheaper Casio ones, they look all blocky and I find that rather pointless. It's worth mentioning though that there are two versions, the Nspire and the Nspire CAS. The latter is NOT allowed, nor is any calculator with some kind of CAS (Computer Algebra System) functionality. Any calculator which is capable of symbolic algebraic manipulation can get you disqualified.
The only problem with the Nspire is that whilst it excels in the graphing department, it's a little lacklustre for basic things. For example, it won't give you answers in terms of pi, whereas a bog standard Β£10 Casio scientific calculator would. It's best in my opinion to use a combination of the two.
I've heard good things about the Casio PRIZM calculators (they have a different name in this country, I think it's CG10). As far as I know, they combine the nice smooth graphing of the Nspire with the natural display usually enjoyed on more basic Casio calculators.
But, there's a calculator you should have before you even consider going graphical and that is the Casio FX-991ES or FX-991ES PLUS. It's a scientific calculator, available for about Β£12 last time I checked and it basically does everything an exam legal calculator can ever do, sans graphs. It will handle matrices, vectors, solve systems of equations up to three variables, quadratics, cubics, numerical differentiation, definite integration and complex numbers. It's really amazing for checking answers.