I think the big three for me are:
1. Flexible on schedule - Airbnb bookings are hard to predict, so they need to be able to come do a small cleaning on a potentially random basis
2. Good communication - I don't have time to write down every detail I need, where the key is hidden, etc. so I require that whoever I work with is a good communicator.
3. Not too expensive - I pass on the cleaning fee, but I don't want to disincentivize customers if the fee's too high.
There are a few more things I look for but that sums it up. Hux's cleaning blog is a solid resources for these things.