If you have the space, software is nice because it’s easier to get the system going again, but the data (your files - music, documents, pictures) and system configuration files (/etc for example) are the most critical. If you have databases set up, learn about their dump commands and add that.
You don’t have to use the same method for everything. My pictures are backed up to another side in a second computer and to Amazon Glacier for $2/month (I’ll have to pay to download them if I ever need it, but I’ll gladly pay if I’m in that situation - those should only be needed if I have a major house fire or something like that). My weekly backups are my /home directories, /etc, /root, a database dump, and maybe one or two other important things.