Good question! There are so many cast ons it can be confusing. Everyone has their favorite, too. For me, and many others, my go to is long tail. It's fairly stretchy and looks nice with most projects.
German twisted is a good stretchy one for necks, wrists and sock tops. The knitted cast on is a good go to one and easy to learn for beginners.
There are a few specialty ones that can be used for ribbing or other things, but they aren't used as much.
The one I would caution you to avoid is backward loop. Except in certain situations like a few stitches in the middle of a project it often doesn't look neat and can cause other problems.