IMO arms(wrist,bicep,tricep) are the most important...cause u
need yr arms for most of other mass gain exercises like deadlift,
bench, presses etc..
i was VERY weak when i 1st started out, most new ppl i see in gym
can start benching with 10kg but when i 1st started out benching, i
couldnt even bench properly a barbell with 0 weights cause my arms
were unstable n shaky...However after a few weeks of arm trianing n
of course more benching i could proceed with weights on the
barbell...