There is no one package to rule them all...
Matlab is virtually the standard among engineers and very good for numerical computations, Maple and also Mathematica compete in the symbolic mathematics arena and they both are quite good (in this case you need to figure out what field of mathematics you're interested into decide which package is better for you). Among people working in biostatistics SAS is also pretty much the standard in the industry but R is pretty much the standard among statisticians of any field...
So as you see, it depends on what field you are working and even the branch of that field. But anyway, my choice is:
FOSS
Statistics: R
Symbolic Math: Maxima
Numerical Math: Octave/Scilab
Commercial
Statistics: SAS, S-Plus, Statistica... Many others.
Symbolic Math: Mathematica/Maple
Numerical Math: Matlab
Though I find that once you are proficient with the FOSS packages you have no much use for the commercial ones and this is especially true for R.