As you are asking for the best solution to import PST file into Office 365 online without losing any data. I have used
Office365Import Tool to import PST file into o365 mailbox perfectly. Really, it did the great job. So, in paid solution
I would like to suggest you this tool to perform PST to O365 migration in few clicks.
know more detail :- http://www.emailsoftwares.com/migrate-outlook-pst-to-office-365.html
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