Surely the safest, easiest, and least painful place to give an intramuscular injection is the glutes!
But why do doctors, for example when they're administering a vaccine, almost always inject into the upper arm! And many times it leaves you with a sore arm for a week! I remember a few years back a friend of mine got shots before heading to Thailand and his arm was killing him for days.
Of all the injections I've ever been given by doctors, all of them were into the upper arm except for the one time I got a Tetanus shot in my ass. I just don't get what they like about the upper arm.
If ever I was going for some sort of vaccine again, I'd be like "Here Doc, gimmie the syringe, I'm sticking it in my ass, I've injected that spot a hundred times"