Well, my thoughts on each of them:
Burai: Burai was awesome. But the downside, he was like one of the few things that actually stood out in Zyuranger (not that it was a bad show or anything).
Kou: IMO, Kou was the first Sixth Ranger in the franchise that was done right. Though he wasn't treated as important as the other five Dairangers, he still helped bring more to the story and help it move along. Plus, he was a key player in Lin's season long story. Oh, and of course, his character was great! :thumbs:
Riki: Riki is okay. Nothing too special as far as characters go. He just comes and does whatever is important and leaves it as that. So nothing bad either. Probably the best credit I can give him is that despite the actor's age, he still manages to pull off being more mature than most of the adults in the show.
Yuusaku: I know a lot of people tend to consider him forgettable, but I disagree. Yuusaku to me was just a Sixth Ranger that was handled very differently compared to others. And IMO, had quite a unique role. I loved how he acted as like an older brother/aloof mentor to the Megarangers which was a contrast to Kubota's fatherly/more strict mentoring. Overall, he was just lots of fun to watch and really helped the Megarangers grow throughout the series.
Naoto: Like most pre-Boukenger Sixth Rangers, Naoto isn't really a part of the team, so nothing changes too much in terms of the dynamics. All that Naoto did was that he brought in more conflict to the show, more to the story themes of the series, and brought plenty of badassness. Plus, it was always fun whenever the Timerangers interacted with him. So Naoto was a pretty good Sixth Ranger.
Tsukumaro: Tsukumaro starts off with the kickass Rouki arc. Now, after he becomes Gaosilver, he goes through the arc of trying to destroy the 1000 years evil to atone for his sins. Tsukumaro is a quiet individual who often doesn't want to socialize with the other Gaorangers, even though they worry for him very much. I know some fans call Tsukumaro as dull, but I think he has plenty of awesome moments, and like Naoto, it's fun whenever he interacts with the team.
Shurikenger: Shurikenger was okay. He was plenty of fun with how he disguised as former Sentai actors and spoke with his broken english. lol. I think the only thing about Shurikenger that I found kinda off was his sudden change to a darker tone in the last 10 eps when the Gozen arc started. The Gozen arc is cool, but it hurt the consistency of Shurikenger's character. Oh well.
Mikoto: Like Yuusaku, I think Mikoto was another very different approach to the Sixth Ranger concept. Mikoto starts off as an actual villain, and remains as one until near the end of the show. And Mikoto does a pretty good job at being a despicable and unforgivable bastard. The whole show has Ryouga trying to change his heart, in which he finally succeeds at in the end. But by then, Mikoto would be killed off, unable to make up for his sins, as much as he wanted to.
Tetsu: I feel like a lot of people tend to ignore Tetsu as well. Yeah, I know he's not the best Sixth Ranger ever, but I liked how he starts off with an elitist attitude, and very soon, gets intrigued by the other Dekarangers and decides to learn from them. Tetsu could've just been this annoying jerk but instead becomes a Sixth Ranger that wishes to explore a whole new world that's unknown to him. Oh, and he crossdresses!
Hikaru: Hikaru to me was the last really solid Sixth Ranger. Like cypsiman2 said, he brings in the role of a mentor to the Magirangers, which is something they lacked for almost 20 eps, due to the "death" of their mother. I also love how despite being their mentor, Hikaru interacts well with the Magirangers and is not only just a mentor to them, but becomes a close friend, and eventually, their brother in law.
Eiji, Hiroto, Miu, Genta, Goseiknight, Gai: Okay, this is where things start to get a bit weird. Before, fans were always excited to see the next Sixth Ranger. But this is where I think fans start getting divided on their presences. Gone was the execution of having a Sixth Ranger be a recurring character that helped support the other five in their growth (Burai through Hikaru). Starting with Eiji, the Sixth Rangers became much more of an addition to the team and were very involved with everything they did. Thus, the team dynamics changed up big time. This would definitely bug a lot of fans. IMO, Eiji was handled the best out of all of them, due to how subtle he was. But he did show signs of what might happen in later seasons. Go-on, Shinken, Gosei, and Gokai all had Sixth Rangers that were meant to be the complete opposite of the team, in terms of silliness and seriousness. Thus, the team dynamics would change big time.
I really didn't like Hiroto and Miu. Mainly cause of how they were blatantly jerks to everyone. Though Miu got better in like 10 eps, and Hiroto got better in like, the last 10 eps, they still didn't really bring much of anything to the show. Seems they just came in to scold the Go-ongers.
Genta I think was better, but he also didn't really add anything to the story. Plus, his introduction was followed by like 20 eps of filler and merchandise. So there was lots of the team doing nothing. But I do think Genta was a fun character for who he was at least.
Goseiknight IMO is the worst Sixth Ranger to date. To me, he was a blatant watered down clone of BullBlack from Gingaman. All his dialogue and actions were quite predictable and his character development kept on resetting. It's like he never knew what he wanted to do. And worst of all, he kept involving himself in the dynamics of the Goseigers. I think the Goseigers truly did have a chance to improve, but Goseiknight just prevented that from happening.
I do think Gai is better than the previous Sixth Rangers from recent seasons. He does bring more to the Gokaiger's team with his knowledge of Super Sentai and helps shake the plots up a bit. Yeah, he's still a stark contrast compared to the rest of the team, but I personally feel like he's a whole lot more subtle compared to the last three Sixth Rangers. While Gai isn't the best Sixth Ranger ever, I think he's definitely a step in the right direction.
