I was the original registrant and owner of the domain name my (former) company website. The company never "acquired" nor did I assign it to the company. The company is now threatening that I should hand over the domain name or else...
Since no assignment was made, am I right in assuming that the domain name belongs to me? or do they have a case?
Your answer can't be found here; you will need to review all contracts and informal agreements (emails, meeting notes, etc) that you have made.
It would be normal for a partnership agreement, employment contract, consulting agreement or similar to contain a catch-all intellectual property (IP) transfer clause. If you signed an IP transfer, then read it carefully. See the canonical intro to IP transfer clauses.
Even if you did not sign an IP transfer, the domain might still belong to your former employer. Just the fact that you were employed brings with it duties -- but which ones and their extent depends on the country.
So, review the agreements you have made, and the laws in your area. And then go see a qualified lawyer.
I personally agree with @Steve Jones -- this isn't worth fighting over.