We still have standard CRM Import tool and now also the new Data Loader Service, which I did not had the time to test it (of course for Online).
The other tools mostly in short list are Scribe either Online or Insight and SSIS in combination with kingswaysoft to connect to CRM.
'DLS, possible alternative for CRM Online'
Still the biggest challenge is how to handle a lot of data, so don't think in thousands, but more in millions of records. This is still difficult, it might be that the earlier mentioned DLS could help you out for CRM Onlime, but I am not sure yet.
The still fasted method at least for on premise or partner hosted would be migration with MS SQL. There is a catch, this is 'unsupported' which means if you break it you have to fixed it yourself. Nevertheless for consultants which very good understanding of database and impact on GUI of CRM it is solid alternative. It will involve more testing to see CRM is still working, but you could increase the time for go live migration with days or weeks.
Of course 'unsupported' means that in case you use this kind of migration approach and you need to get Microsoft Support involved it would be a challenge. So be aware that you check your data integrity while using the MS SQL. In my personal experience its lot faster and could save a lot of time also very handy for migrating annotations.
To be supported or not to be supported, that's your choice!