WinMerge is the best diff tool I’ve found. I haven’t used it for merges, because I prefer four-pane merges (original, left, right, and final) and it doesn’t support that. For that I use kdiff3, though I’d love to find a replacement...

WinMerge is open source - maybe free software, I don’t know. Development halted for years but was picked back up. It’s so solid that I haven’t upgraded in a decade? or more.