Oh nice! I've always wanted to take pictures up in a balloon!
Only one lens?? well... Just my thoughts: UWA, telephoto and maybe a tilt shift? Depends on the scenery.. You could get some interesting shots of the landscape below.. Making them look miniature? I would take those kind of shots with the TS. Wide general purpose for maybe inside the balloon.. Occupants/self photos. Tele for other balloons and their occupants. And for things far below? If I was forced to take one lens, UWA. Have fun! Oh and wear a strap like a blackrapid. Lol
5D3, SL1, EOS M, 28mm f1.8, 10-22mm f3.5-4.5, 60mm f2.8, 40mm f2.8, 85mm f1.8, 300mm F4 L IS, 70-200mm F4 L IS, 24-105mm F4 L IS USM, 50mm f1.8 STM, 14mm f2.8 Samyang, Sigma 150-600mm C, 430 EX II, eos m lenses, bags and too much lighting equipment.