For a business critical web service I would look at a decent dedicated server provider, unless you really need the API and scalability features of AWS. OVH are

For a business critical web service I would look at a decent dedicated server provider, unless you really need the API and scalability features of AWS. OVH are great, I use them my self, however they are cheap and that's for a reason, the hardware isn't server grade and the support is almost Nill.