Viktor Adler/Second Master Org in Power Rangers Wild Force
Power Rangers usually had some villains that stood out... and I think Master Org is among them. While Zedd initially appeared as scary but this guy had some some things that weren't originally allowed in Power Rangers. Which of course, this second Master Org was actually a pretty much vengeful human which added a dark touch to Wild Force, a show that otherwise got criticized for its inadequate acting. What we slowly discover is the fact that he is really human who had cast away his humanity, in exchange for revenge and Cole had a huge part of it.
Looking at him, he's not your average Power Rangers villain and just a few more tweaks he could have also been a threat to the Gaorangers as well. I mean, I'm glad he isn't all that sissified for U.S. media. Nope, after he was overshadowed by Cole's father, he even fell outmatched when he was once in love with Cole's mother... well instead of being just an ordinary sore loser he isn't. In the jungle, he got the Org remnants, swallowed them and you guessed it, acquired Org powers while losing his humanity. He even managed to control Jindrax and Toxica for some time until he was discovered. He is no Orcus on the throne like Zedd, he really knows how to tackle the enemy. He was pure nightmare fuel with an insane, cruel mind.
This of course was his hatred for Cole was when he discovered that the latter was the child of his once best friend turned bitter rival. He killed both Cole's parents after all. He also fooled him and the other Wild Force Rangers into a dangerous ordeal. He wrapped the others around in gross killer slime that would have killed them. Challenging Cole, it's one of the best scenes in Wild Force. He did revert back to plain old Viktor Adler, overthrown by Mandilok but that was NOT the freaking end of him yet. Either way, he was probably already slowly getting rid of his humanity.
Unknown to everyone but the audience, Viktor Adler had stopped being half Org to becoming a whole one after he supposedly died. What's pretty amazing is that now as a real Org, Viktor Adler still possessed his memories and even remembered how he was betrayed by Jindrax, Toxica and Mandilok. He sent Onikage to trick them all, to prepare for his return and he even blasted Mandilok in one blast (in Gaoranger, Mandilok's counterpart Rasetsu was destroyed by Gao Kentaurus). He even manipulated everyone getting ready for his return which eventually pushes itself to the finale.
Well he did copy Gaoranger's Senki as his final form which for me, is a plus point of Wild Force against Gaoranger though I still love both shows. And you could think that he goes freaking awesome when he also destroys Animaria, destroys the Wild Zords and goes on a total rampage. Well he was later destroyed by the revived Wild Zords and the Wild Force Rangers, ending his insanity permanently. It's also something Cole still forgave him even after all that!