Best free software?

I had my old boss contact me the other day wanting me to join his team. I haven't been into GIS in a while, I took a different path in my career but this position he is offering is way better than where I'm at now. I am looking for some free software to kind of "shake the rust off," if anyone has any recommendations? Also some tutorials or anything you would find helpful for me to get back into it? Thank you so much for your time