Quality of service is hands down the most important aspect of choosing a host.
Think about it, the quality of the web hosting you can provide as a reseller is only as good as the host who is offering it to you. Thats why when I started out, I sampled many reseller's. In the end, I chose a host with ZERO overselling, servers which weren't overloaded and with low loads. I could tell a large level of difference between these hosts when sampling them, and went for the best one (which was also more money).
Second would be quality of support, especially if depending on a end-user set up.
Quality comes at a price, and its both sad and comical to see people complain about their $5/mo reseller account that provides poor service.
Regards,