Quads are my favorite spot. I mainly use delt quad ventro glute.
I think 5/8 27g is the best, but .5 inch slins are a close second. With the shorter needles make sure to simple the skin (push the needle deep so you basically make a crater shape in the skin)
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But the 1ml barrel is required for the 27g to be fun. If you have 5/8" 25s they are useable with the 3ml barrel but still very slow compared to a 28g slin