BBEdit is the bomb on the Mac. Great text editor as well for any application or web work.
I use BBEdit for a lot of things, for websites to writing my code.
I was just trying to do a compare with a parsed output of a log file, one from Python on the Mac and one from Visual C on Windows, a bit over 10,000 lines. BBEdit puked on it, wouldn't even give me an output (Find Differences...).
Beyond Compare 2.5.x (Win) felt like the fastest and I liked its output. This was using VMWare Fusion and Win7 on my 2011 15" MBP.
I can't live without Total Commander on Windows. Its Quick Compare, which I use all the time, had terrible results. My buddy's Visual C output was odd, so it couldn't handle it.
DeltaWalker (v1.9.8) did a decent job, but was a bit pokier than BeyondCompare. I wish it would give a better indication that it's doing something.
Kaleidoscope (v1.1.6) totally puked on this as well. It died trying to load the second file.
FileMerge (XCode) also never came back after trying to load the second file.
That was all I tried this morning for a relatively straight forward compare. I think I may spring for DeltaWalker, since it's at least Mac native and actually worked. I like its output window better than BBEdit's.
[edit: I didn't try
Changes since some comments out there suggested that it isn't being actively worked on anymore. I don't know if that's true, however. I also didn't try
Araxis Merge because it's a bit more expensive ($129).]