First of all, Identification of the Critical business applications, which will be finally migrated to the cloud need to be identified and accordingly partner to be assessed. Prime concern would be the Data Security, Data Governance and Service Dependencies. So, all kind of Security Certification is necessary and the team should comprise of certified professionals. They should comply with Industry Standards and Quality frameworks, follow industry best practices. It should match and support company’s own security policies and processes.
Then comes the Overall portfolio of services that partner can offer. Here Company stature, business volume, similar industry experience are key important factors along with the Contract Terms, Commercials, Exit Policy, SLAs need to be looked into.
In case migration to a hybrid cloud, where the infrastructure is a mix of on-premises, private, and public cloud services, the partner should have experience setting up this type of complex environment.
Right blend of technical expertise, Project management and services are necessary keeping in mind Zero Business Downtime and migrating a large amount of data to the cloud. Partner's disaster recovery provisions, processes and their ability to support for any emergency.
Vendor lock in is one of the very important thing to be looked in to so that partner should have minimal use of proprietary technology that will create problem for any future reverse migration.