Quads have potential for issues when you inject the top/front of the quad, and what we mean by "issues" here is hitting a nerve and experiencing distinctly unpleasant sensations as a result.
You will not have issues injecting the outside side of the quad (90 degrees off from the top center if you were looking at a cross-section of the leg, 9 or 3 o'clock on a clock). It took me a long time to figure this out actually, as the typical recommendation is to inject the outer third of the quad, which I took to mean injecting the top of your thigh as viewed from a seated position, towards the outer edge. Nope, you'll hit nerves doing that, as I discovered repeatedly. You want to inject the SIDE of your leg, not the top.
Delts are a great choice for most people, but mine leak oil for some reason, with any needle shorter than 1". I don't understand why, and it is the only injection site where I've ever experienced leakage. A more common issue some have is insufficient flexibility to reach across the body to self-administer a delt injection.