quads and delts.

Deltoids are easy -- there's pretty much no fat over them if you're a healthy weight.

Quads are a bit more problematic... you need to find where the subQ fat is really thin. Generally, this is starting on the side above the knee.

I tried biceps a few times and got a few ugly bruises. I think deltoids worked when I used them. When I get some thicker gauge needles, I'll give them a good test. I inject in my triceps pretty regularly, but I'm not going deep. When I do quads, it's usually through jeans/pants, so not very deep.