When it comes to manual merge of text files, Beyond compare is best tool I ever liked. This tools really helps to see differences with colors and I can easily push code from one file to other. other best thing is it compares folders.

I love the ability to compare code before and after to ensure nothing else changed

Easy to compare 2 different texts, folders with files, other data, even hex, picture, registry, versions... And merge text. It shows clearly the similitudes and differences between the 2.