Long Tail Cast On
The Long Tail Cast On is a great multipurpose cast on. It's stretchy enough for many different kinds of projects, and once your fingers memorize the pattern, you'll never forget it!
I like this tutorial from WEBS, which includes a tip for determining how long your tail needs to be so you don't run out of yarn in the middle of casting on.
One thing to keep in mind with the Long Tail Cast On is that how much room you leave between stitches as you cast them on will determine how tight your cast-on edge is once you start knitting. It can be helpful to place your right index finger in front of the last stitch on your needle as you cast on the next one and only snug the new stitch up to the opposite side of that finger when you need to make sure your cast-on has a good amount of give.