Most Power Rangers seasons have acting that ranges from average to mediocre, or in Samurai's case, just plain terrible. However, there have been a handful of seasons that have had descent and even good acting. Here are the seasons, in no particular order, that have descent to good acting.
Power Rangers Time Force
Time Force is considered by many to be the most well-acted series ever in Power Rangers, and with good reason. Strong performances were put on by its two leads, Jason Faunt (who played Wes) and Erin Cahill (who played Jen). Also adding to their acting talents were two Hollywood actors Edward Albert (who played the Wes's father) and Vernon Wells (who played the main villain Ransik). I also feel Daniel Southworth, who played Eric Meyers, put on a good performance as well. However, I feel that this series tends to get overrated by many due to lacking in character development for half the cast and copying most of its story from Timeranger.
Power Rangers Ninja Storm
Many power ranger fans tend to credit Ninja Storm for having good acting. I personally think the acting in this season is a bit overrated. Perhaps it's overrated because it's one of the few Disney era seasons where there is very little accent slippage from the New Zealand actors. It's not bad. I did like the performances by Jason Chan (Cam), Sally Martin (Tori), and Grant McFarland (who played both sensei-Watanabe and Lothor). The rest, however, I felt were fairly average.
Power Rangers Dino Thunder
Dino Thunder is another season that tends to get credit for good acting. There were some good actors like James Napier (even though his character, Connor, is a dimwit), Emma Lahana (who can sing as well), and Jeffrey Parazzo(who played Trent).
Power Rangers SPD
SPD is another season that gets a lot of praise for good acting. It was also the last season before Samurai to use mostly North American actors. There were good performances from the entire cast, although personally I don't think they had the same charm as their Dekaranger counterparts. Despite the excellent acting in SPD, the series had some writing problems, particularly with the Omega Ranger scenario and pacing issues with many filler episodes taking place during the middle of the season.
Power Rangers RPM
RPM I think had the best acting of any series. The core 5 members all put on solid performances, as well as some of the supporting characters such as Dr. K (played by Olivia Tennet) and Colonel Trueman (played by James Gaylyn). Personally, I think the best performances were put on by Milo Cawthorne (Ziggy) who was great in a comedic role, and Ari Boyland (Flynn) who showed a very convincing Scottish accent.