Excel.

Microsoft Excel is amazing.
The things you can do with it are endless and it is very useful.
I know this because I once worked as a Trainee at a company for their Accounting and Finance section and these spreadsheets were the only things that I saw day in and day out.

Some of the tasks were quite simple and normal because I'm used to it.

However there were some other tasks qhich got me stuck but I learned how to deal with it and so everything was good.

Excel is amazing and I will use it more soon and I know for sure this won't be the last time I'll be using it.