If you
must, bari sax. A guy in our community band has an exquisite BBb bass sax, Even though it sounds great, frankly, it's even heavier than a tuba, and there's nothing written for it anymore. He tried to play bass clarinet parts, or even contra clarinet parts on it, and the way those parts are scored, it just got muddy on the bottom. If he tried to play some of the parts "as written," instead of bringing them up an octave, there were times he was below the tubas, and it really sounded strange. It's too bad, because he is an excellent player. He finally gave it up after a couple of years and bought a Selmer contra clarinet to play instead. As bloke said, and from being spoiled in a high school band that had not only alto and three bass clarinets, but all three of the others as well: bari sax, Eb contra clarinet AND BBb contra clarinet, the Selmer sounds great. But outside of concert band, there is really no practical application, if you could ever call a contra clarinet practical.
So, with bari sax, you can play in concert band, fulfill a definite requirement in jazz bands, which most, as we all know, don't use tuba, opting for upright or electric bass instead, and...hold on...in a pinch, you can play Eb tuba brass band parts.
Please stay away from bassoon. Unless you pick one up at an early age, have fingers that can spread wide for the keys, and have a significant overbite to deal with the double reed, it just won't work for the average tuba embouchure. Unlike a saxophone, it's not an instrument to pick up casually. We have, (insert disdain) three in our community band, and the director finally had them put crocheted caps, kind of like miniature open weave golf head covers, over their bells so they would finally blend.
For other options,
http://www.contrabass.com
I don't know what would happen if you showed up with a sarrusophone.