glute= least pain and a higher tolorence to larger doses of gear
thigh= ok, but its hard for me to push the pin in. I dunno why, the muscle gets tight.
delt= great site to hit, moderately painless, but mine like to bleed a lot when I hit them
pec= painful to break the skin, but I like it for 2cc or less, easy access
bicep/tricep= very painful for me at least, but they are the easiest access, and it looks cool.
lat= painless, great site to hit, but hard to reach. I would encourage only those with mature lats.
calf= never hit, because thats absolutely rediculous, its too small
trap= scary for me, never hit, too many horror stories
hams= too many small muscles, ie. ham STRINGS
I think that about wraps it up. The best is without a doubt is the glute, however, my favorite is the pec.