Safe and dry is important, and so is light and shade.
Depending on where your run is, green Onduline (or Wickes equivalent) will give some of the shade they need in hot conditions, but will make it a lot darker in the winter months.
Clear plastic corrugated, which I have, does let in a lovely lot of light but gets hot in summer. In my run this is mitigated by large trees which provide shade from early afternoon onwards, and also by willow screening fixed to the mesh sides on the South and West facing sides. This lets air through and creates nicely filtered shade, also some protection from wind and rain blowing in through the mesh at the sides. You may need to provide some vertical shading as well, even if you use green Onduline or equivalent.
So it's a compromise, depending on which way the run faces in relation to the sun. They must have shade, chickens cope very well with cold conditions but get distressed if exposed to hot sun with no way of cooling down. Think 'woodland edge', which is the sort of sheltered environment they would ideally choose to live in.
We just used ordinary bog standard clear plastic corrugated sheet from Wickes, and so far this has lasted well, hasn't got blown away in any of the gales we've had in the past 4 years, fixed correctly to a good wooden framework across the top. If and when it does, it's cheap and easy to replace.
You need a gutter on the low side, as otherwise the rain will run down over the edge and blow in to the run sideways.
