It's no secret even among hardcore fans of Power Rangers and Super Sentai that neither show is an example of good acting. In fact, one of the biggest criticisms against either series is the lack of "quality acting," and it's noticable in some seasons that the acting is outright terrible. It is true that many Power Ranger and Sentai actors and actresses begin and end their careers with Power Rangers/Sentai because they can't get any other roles. While by my own admittance, neither show has what very many people would call "great acting," I also argue that you don't need great acting to make a show, series, or movie good. Don't get me wrong, acting should count for something, but as long as the acting is sufficient and not outright terrible (like Wild Force and Operation Overdrive), the quality of the show, the characters, and the actors/actresses that play them can still be very much enjoyable. In fact, sometimes a show with good or great acting can still be mediocre or awful due to bad execution, story writing, directing, special effects, exc.
One such example from Hollywood would be Arnold Schwarzenegger. Schwarzenegger by no means has Oscar award winning acting talent, and for that matter he wasn't that good of a governor either. However, Schwarzenegger is still a very enjoyable actor to watch. Not because of his acting skills, but because of his big physic, charisma, and because his acting is sufficient, he makes a great action hero. The fact is, 90% of all audiences don't recognize bad acting unless it's painfully obvious.
Sufficient Acting, Good Shows
In Power Rangers the two season's I'd like to point out in particular are Mighty Morphin and In Space. Mighty Morphin especially had mixed acting. Overall, I thought Amy Jo Johnson was as good of an actress that's ever been on Power Rangers, David Yost, Thuy Trang, and Johnny Yong Bosch were descent actors, Walter Jones was ok. Austin St. John and Jason David Frank weren't very good actors initially (although the evil Green Ranger laugh was chilling) but they were both great martial artist which made their fight scenes awesome to watch, and their acting did improve quite a bit over time. Despite the mixed acting during MMPR, the acting did improve overtime and the actors were good enough to make their characters likable. And as mentioned above, hardly anyone is going to recognize bad acting and especially not young children which was the bulk of the demographics watching MMPR.
In Space had better acting than the early days of MMPR, but still didn't have anyone with the same acting quality as Amy Jo Johnson or even Johnny Yong Bosch. However, In Space is a great show, IMO the best PR season ever. The acting was sufficient enough to get the job done and make the characters enjoyable. What really makes In Space great was the quality of the story, the character development, and the awesome villains that this season had.
Sentai, however, has many numerous examples of "sufficient acting, good/great shows." This includes Jetman, Timeranger, Hurricanger, Boukenger, Gekiranger, Go-Onger, and Shinkenger. All of these series, while some have other obvious flaws, tend to be well written, have solid characters and villains, tend to have good stories, and excellent humor (except for Timeranger which doesn't intend to be humorous). Some of these shows have had some mildly good and successful actors. Timeranger had Masahiro Kuranuki (Sion/Time Green), Hurricanger had Nao Nagasawa (Nanami/Hurricane Blue), Boukenger had Haruka Suenaga (Sakura/Bouken Pink), and Gekiranger had Kazue Itoh (Miki of Gekiranger). However, for the most part, these series had PR-like sufficient actors that were good enough to get the job done.
Good Acting, Lower Quality Show
Then there are seasons with good acting but are lower quality seasons compared to others. One such example is Power Rangers SPD, which has very quality acting by PR standards, perhaps the best ever for a PR season. However, YMMV greatly on rather SPD is a good season or a bad season. IMHO, I think it's a descent season. It has many good points and a few interesting characters and story lines, but is still filled with many flaws. The most noticeble being the Omega Ranger scenario and the fact that SPD's story really lost steam as the season progressed. For me, while not a bad season, it's good acting is not nearly enough to overcome the other flaws this season has to make it a great season.
Bad Acting, Low Quality
Then there are seasons like Wild Force, which has bad acting even below MMPR's early day standards. When I say Wild Force has bad acting, I mean even the 90% of all audience members who normally can't recognize bad acting will notice the terrible acting of this series. Particularly by Ricardo Medina (Cole Evans/Wild Force Red) who overacts and is quite often prone to chewing out the scenery, and Ann Marie Crouch (Princess Shayla) whom both overacts and underacts in the majority of her scenes. Besides the terrible acting, there are some rather very cheesy aspects of Wild Force (cheesier than MMPR IMO) that make the viewers cringe, such as reading the story of Animarium from a childrens book, singing a god awful song to the zords, defeating an enemy with a zord used like a soccer ball in a CGI soccer stadium, and the ultra-preachy environmental message from Animus. Many of the guest actors/actresses are also aweful in Wild Force. Still, unlike the next series on this list, Wild Force does have some redeeming qualities, such as the team-ups, the story behind Cole's parents, and the overall dark story behind the series.
Like Wild Force, Operation Overdrive also has considerably noticable bad acting. Almost the entire cast is terrible but most glaringly are Gareth Yuen (Dax Lo/Overdrive Blue) and all the voice actors for the villains and the Sentinal Knight. Unlike Wild Force, simply everything is terrible about this season, including the dialog, the extreme overuse of explosions (Kalishplosions named after writer/producer Bruce Kalish), and very spotty writing. Not even the anniversary team-up, Once A Ranger, was enough to redeem this series.
For Sentai, one series that could be argued to have below par acting is Goseiger. Almost all of the main characters are pretty wooden. However, this can also arguably be blamed on the writers/directors, because most Goseiger characters have pretty bland personalities and the series overall is very weak on character development.
Good Acting, Good Show
Lastly, there are some series where the quality of acting makes an already good show, better. For Power Rangers, the most noticable examples are RPM and Time Force. Unlike previous seasons, the producers of RPM had handpicked already well established Australian and New Zealand actors that had previous experience in other series. All of these actors could show a wide range of emotions. Among the best IMO were Ari Boyland (Flynn), who displayed a convincing Scottish accent, and Milo Cawthorne (Ziggy) who was incredibly funny.
For Timeforce, the performances by Erin Cahill (Jen/Time Force Pink), Jason Faunt (Wesley/Time Force Red), Daniel Southworth (Eric/Quantum Ranger), and Vernon Wells (Ransik) were particularly good and believable. Overall, Time Force is a pretty good series, but personally I think it's on the overrated side due to copying the bulk of it's story from it's Sentai counterpart, Timeranger, and for the lack of character development given to some of it's other main characters such as Lucus, Trip, and Katie.
I may be biased since Dekaranger is my favorite Sentai but I also think Dekaranger had the best acting among any 2000's Sentai. Like RPM, all of the actors were able to show a wide range of emotions, portrayed their characters beautifully, and many have had somewhat successful acting careers. Ayumi Kinoshita (Jasmine), Ryuji Sainei (Ban), and Tsuyoshi Hayashi (Hoji) have appeared in quite a few Japanese movies and TV dramas, while Mika Kikuchi (Umeko), Tetsu Inada (voice of Doggie Krugar), and Ryusei Nakao (voice of Agent Abrera) have had successful voice acting careers in anime. As far as the show itself, it's quite awesome. The characters are lovely, the music is cool, and so are the fight scenes.