Honestly, if I had to choose my "main" instrument, it would be the piano. As far as studying harmony and theory goes, the piano is pretty much the be-all-end-all of everything. Chords, interval relationships, scales, are all so simple and beautiful to visualize on the piano, and I honestly doubt that without my knowledge of the piano, I would've had the patience to go on to learn drumming, guitar, bass, etc...
The great thing about multi-instrumentalism is the ability one gains to apply techniques from different instruments to alternate uses. i.e. hand percussion has actually helped my piano technique substantially, and knowledge of the the saxophone helped me to understand good phrasing on the bass. etc...
Good luck!