What are the best tools for migrating Exchange and IMAP to Office 365?

Hello all, I'm looking for some recommendations for the best tools for migrating Exchange and IMAP email accounts to Office 365.

Our company is in the process of switching over to Office 365 and I need to ensure a smooth transition for all of our emails, contacts and tasks. we are trying to migrate from OVH to 0365. Writing a program to do this is possible, but so far it has turned out to be more complex than expected due to the lack of good libraries available to accomplish this task.

Some of our accounts have hundreds of gigabyte so therefore something with a delta migration would be amazing.

Any suggestions or experiences with certain tools would be greatly appreciated. Thank you in advance!

PS: Im quite stunned that the integrated migration assistant in office 365 is soo bad, even when connecting to recent exchange servers