Definitely the trombone...
I suspect most people will tell you 'bass' and site reasons such as anchoring the band, administering the groove and 'just because it's the coolest'.
When it comes down to it though, a lot has got to be to do with what you've got a vision to do (when you see a band, which instrument would you most like to be playing) and opportunity (is there a bass you can learn on, do you know anyone who can teach you, has your band got too many guitarists, etc).
Those are probably better factors to consider in making a decision than asking a bunch of self-confessed bass-aholics
Wulf
