aherm... 
Well, as a percussionist it's essential that I'm well versed on all of the instruments in that category...  maybe with a few exceptions (I wish knew more about hand drums, steel drums, etc).  In some ways I feel I'm lucky that I've haphazardly fallen into that "instrument", because I've always been good at picking up new things quickly. 
So what would my secondary instrument be, then?  I guess piano... I took lessons for quite a while.  And should I be discerning between jazz and classical?  I'm better at jazz piano than classical piano.  Or should i be considering jazz a secondary instrument to classical as well?  I also dabble with flute, trumpet, and sax, but for no reason other than I play them when at the computer when they're in an arms reach. 
I guess if I were to speak within percussion, my "primary" instrument (meaning the one I'm best at) would be marimba/vibraphone, followed by snare drum, followed by timpani.  God I need to play more timpani...