The most basic system will just have the:
Main
Updates
Security Updates
repositories, but I recommend following the "normal" route below.
just a working system..
If you just want to get back to a working system then check:
These are the most common repos that people use.
I, personally, would also enable the:
repositories, but those are entirely optional.
3rd party repos..
If you aren't sure or just want a working system, leave these alone. You won't need them. Otherwise, go ahead and check any software you will want to use. These repositories are good for getting software that isn't in any of the other repos or software newer than those in the repos.
The one exception to this might be the "Oracle Java (JDK) Installer PPA" repository, which is the best (IMO) way to get the Oracle Java JDK on Ubuntu. If you aren't going to develop Java apps or do anything else that requires the Java JDK, then the above applies here: skip it. If you need it later, it isn't hard to get.
Note that you will have to install the software to actually "get" it. This just adds the software to your sources so it is possible to install it.
When you're done, click "Generate List". You'll need to put the generated list in the /etc/apt/sources.list
file, replacing anything already present in the file. If you selected any 3rd party repos you will need to add the GPG keys. Follow the directions in the "Getting the GPG keys" box.