Depends on how close you fly to the tandems, it needs to match your video too. I've seen excellent results with everything from a 10.5mm fisheye to a 20mm lens.
I really don't like the kit lenses, but some people manage decent photo's with those Generally fixed is better than zoom but the sigma 10-20mm gets good reviews too. I personally dislike the fisheye effect but fisheyes are cheaper and lighter than straight wideangle lenses.
Most common is anything from 24-28mm..., but depends on how you fly, if you're trying to be artistic, and so on. Keep in mind if you are shooting with a DSLR without a full frame sensor you will have a cropping factor that will magnify the lens you are using, so that will affect which lens you need.
compatibility with your video lens is the most important! I think the wider the better, find it easier to fly close to the tandem and always keep them in frame. Recommend.3-.42 (video) & 16mm fisheye from Zenit (cheap!) ...
Generally fixed is better than zoom but the sigma 10-20mm gets good reviews too. I personally dislike the fisheye effect but fisheyes are cheaper and lighter than straight wideangle lenses. 
