I've done all three hikes, and comparing them as a round trip day hike, R2R2R is the hardest just by virtue of the greater distance (around 45 miles) C2C2C has almost the same total elevation gain and water is in shorter supply, but still it's only 31 miles. Whitney main trail's difficulty is the altitude and if you're not ready for it, then it can be just as hard. But assuming you're at least somewhat prepared for the altitude then it's easier just because it's shorter and less climbing.
