I personally like both WinMerge and BeyondCompare. I've used both, never really had issues with either of them. I've also used KDiff on linux, diff (the command line) on linux, and a few lesser knowns that I can't quite remember the names of.
So pick either one, try them both, and then go with the one you like best. But I'll at least say, both were acceptable in my usage of them (and continued usage of them).