Dr. Charles Ashford

The transcript for when Angela tells Alice of her father making the T-virus and anti-virus make it very clear that Ashford created it and then the Umbrella corporation took it from him.

Angela: My daddy. My daddy made it for me. He's sick. And someday, I'll get sick too. He just wanted to stop that. When I was little I had to walk on crutches. They said I'd never get better, just worse. He found a way to make me stronger.
Alice: The T-virus.
Angela: Then they took the invention away from him.
_{Resident Evil, Apocalypse}

Whereas, in The Final Chapter the transcript tells us that James merely just discovers it, i.e. takes it from Ashford.

Marcus was driven to save her. But the odds seemed impossible. And even as he worked desperately to create a cure, the young girl's father would record his daughter, her voice, her likeness, saving her for posterity. But then a breakthrough came. Marcus discovered the T-Virus.
_{Resident Evil, The Final Chapter}