I bought an fx-P401 either for my GCSE's or A-Levels (I don't remember which) but I've had it and used it for many years since. I loved the fact that you typed the calculation you wanted (brackets included) then hit 'exe'cute to perform the calculation. It performed the calculation in the order you expected rather than having to second guess order of execution and had all various standard casio functions (include hex/binary/decimal modes).
It was advanced enough to do what I needed but not too advanced so it was allowed during my university exams. (eg the FX-1000G graphical calculator that replaced it and was banned!)
I still use it now in work, I'm not entirely sure how many years ago I replaced the batteries but they keep going and going!