I've been searching for the good hosting provider that could offer scaleable service for multi-store site, I noticed that there's one called MageStack offer easy setup and easy scale, just wondering if there's someone has any experience with MageStack, it would be great if you could share some of your comments related with MageStack.
I'm also finding good service provider that could offer AWS setup for multi-store site (multiple web server, load balancer, nginx + php-fpm, centos 7, percona db all use different server), it would be great if you can share your experience with some of good service providers.
There are several good providers out there that have been in business for years. My advise is to pick 3-5 hosts and send them a "pre-sale questionnaire", i.e. what can they offer, how are their support, ask for references, etc. Some will even give you a free trial period to test out their service. This should give you a better idea what to expect from each host.
Also keep in mind that if you choose a IaaS provider like AWS you need a sysadmin or someone that knows Magento very well in order to set it up and maintain the infrastructure for you. If you don't have these skills yourself or in a partner, it might be a better choice to go with a managed provider that takes care of all the technical aspects for you.