Cello - love the sound.
I've dabbled in it but never given it the attention it needs.
I love the sound of the cello too but for whatever reason I just can't ever get the hang of playing bowed instruments or brass. I envy those people who can pick up just about any instrument and make music with it.
Assuming I had the capability of playing them to a good level it would be either the piano (for the vast repertoire which includes some of the greatest music ever written) or the oboe (because of its beautiful, plangent sound).