Are you talking Azure Devops Services or Azure Devops Server?
Do you mean which is the best license for you?
The pricing and features are laid out here https://azure.microsoft.com/en-gb/pricing/details/devops/azure-devops-services/ Which suits you depends on your requirements really.
For many people simple starting with the Basic Plan works fine. I believe MS recommend Visual Studio Enterprise for more than 500 users (and that is developers, not end users) so it really does come down to "are you an enterprise?"
Hope that helps