One that does NOT ONE DAMN THING to detract from the "actual" Power Rangers.
I'm incredibly anti-copyright law, but even to me that's obviously not true. Every time a certain fictional character is used, people's perception of that character changes. By showing the Power Rangers this way the video's creator is changing our perceptions of the Power Rangers.
All it takes is just a single person:
1) seeing the video
2) associating (subconsciously) the brand with darkness
3) deciding to buy a gift of Legos instead of a Power Rangers toy as a result
and then clearly the video has detracted from the "actual" Power Rangers (in the sense that their IP's owner will sell one less toy because of the video).
Don't believe me? Imagine if everyone was legally allowed to use Mickey Mouse. There would Mickey Mouse snuff films, Mickey Mouse versions of 50 Shades, Mickey Mouse promoting drug use, etc. If enough of that stuff exists kids would see it and would never look at Mickey Mouse the same way again. That absolutely detracts from Disney's ability to keep Mickey a wholesome character that they can make tons of money off.
Now, should Mickey be in the public domain by now? Absolutely (screw you Disney). But let's not pretend that if anyone can make any version of any famous character they want that it won't have any effect on how people see that character (and thus detract from the original author's vision of them).