i inject in the glutes... i like the delts but someone has to do that for me so i usually do the glutes b/c i really dont like to ask all the time. lots of people inject in the quad. i have never done so but sounds pretty easy to get too .
Glutes,delts,triceps,quads,and calves....all of which are really easy to do in my opinion and relatively painless.
never tried hitting my biceps or pecs that shit is a little too fucking sketchy for me.