If you are not divorced by Dec 31 you can still file Joint. Joint is usually the best way to file if you both agree. You can lose several benefits by filing as Married filing Separate. You can't file Single until you are divorced.
Or one of you can file as Head of Household if you lived apart for the last 6 months of the year and have a qualifying child. Then the other spouse has to file MFS.
See FAQ on Head of Household
https://ttlc.intuit.com/community/family/help/do-i-qualify-for-head-of-household/00/25539
