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...