What is most important software for of Electrical Engineering?

It is hard to say what the most important software package is as each application will equally be as just as important as the next package you may use. There was no mention of what the software is intended for, so it is again hard to give you a specific answer without knowing what game you are playing. I have used various software packages for load calculations, communications and modelling systems at FAT level, but most are exclusive for the job in hand. Sorry I cannot elaborate much further and I hope you get the answer you are looking for.

I think PROTEUS is the most basic and important software of Electrical Engineering.