Unity + Visual Novel Toolkit = 100% Free: https://assetstore.unity.com/packages/templates/visual-novel-toolkit-free-9416
Then there's also Ren'py https://www.renpy.org/ which is arguably the most popular visual novel engine, therefore would likely be the easiest. Easiest in the way that if you run into any issues, a simple google search will bring up a solution to an issue you're having that likely someone else would have also already had before.