We would like to have an email sent to the contact on the Case when the case owner changes letting them know who is now working on their case. I would like to automate this based on owner ISCHANGED. What would be the best way to do this? WF vs PB vs Flow vs Trigger
The "best" way is usually subjective.
I think it kind of depends on what your implementation looks like.
If you use email to case, you'll want to not only send an email but create an
EmailMessage record. In this case I think a trigger is your best bet. To make sure the email reply is attached to the case you'll want to make sure you include the case thread Id in your email. The best way would be to create a field on the case that generates the thread Id so you can add that field into an email template.
Formula should look like:
"ref:_" & LEFT($Organization.Id,5) & SUBSTITUTE( RIGHT($Organization.Id,10), "0", "" ) & "._" & LEFT(Id,5) & SUBSTITUTE( Left(RIGHT(Id,10), 5), "0", "" ) & RIGHT(Id,5) & ":ref"
Otherwise a PB/WF should work fine.