What do you think are the most important factors to consider when picking an MFA solution?

1. Usability and User Experience
2. Integration Capabilities
3. Scalability and Flexibility
4. Management and Administration
5. Reliability and Availability
6. Cost and Total Cost of Ownership
7. Compliance and Industry Standards
8. Vendor Support and Reputation

Session length, ease of use and unlikely for an accidental acceptance (not just a yes or no pop-up, put in a number)

Realize that, as Roger Grimes writes in ‘Hacking Multifactor Authentication’, that every MFA solution can be hacked. They are far from bulletproof. And it is critical that they be configured correctly.

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I would look for tools that are phishing resistant - MFA is frequently bypassed by session lengths that are too long. I would evaluate tools that are FIDO 2 aligned. 

Having been on both the end of using a company MFA and also putting in one:
* compatibility with existing solution(s)
* secure
* multiple device options for users, a few % will not have smartphones. 
* transparent vendor (esp. in relation to security issues)
* ease to use

Other considerations:
* SaaS based
* Part of greater SSO/directory solution
* ease of integration and troubleshooting