The best version was the movie version by Mr. Big followed shortly by Masaaki Endo's version.
The first season was corny beyond hell and the video quality has degraded horribly. Season 2 had one of the best villains in any series ever plus better stories overall (even if it resulted in the whole British-controlling-California thing).
Season 3 was pure awesome from start to finish. Zedd became less scary but was arguably the most effective villain ever until RPM. Zeo had several good storylines but many were forced to be dropped (such as Billy becoming the Gold Ranger and Tommy's brother).
Turbo was awful. It actually got better when the new Rangers came in, but the show was still awful.
In Space was arguably the best season. It had several great characters, a great setting, tons of development, and a mature (for PR anyway) storyline.
Lost Galaxy had a metric ton of production problems every episode yet still managed to make an excellent series, so much so that this was the PR series Saban pegged to launch Vortexx with.
From then on it goes up and down. Lightspeed Rescue had good concepts, but pales in comparison to the last two series or its following season, though Linkara's reflection on the series helped put it in a more positive light.
Time Force had the most mature PR storyline and stuck closely to the original series. It also had Vernon Wells as the main bad guy.
Wild Force is very, very cheesy and introduced CG Zords. Most people consider the best episodes to be the Time Force crossover.
Ninja Storm is even cheesier and even though the main bad guy had some successes, he still seemed lame. Cam was awesomeness though.
DinoThunder brought back Tommy and while the story was weak as hell (not helped by the White Ranger clone and Jason David Frank not appearing for about half the season) but the bonding of the main cast and the scary villain kept fans coming back.
After that the show was taken over by Bruce Kalish and the show went through troubles. SPD was OK, Mystic Force and Operation Overdrive sucked (save for the crossover 2-parter in OO where Johnny Young Bosch came back), and Jungle Fury was pretty by-the-book.
RPM is considered by many to be the best season ever. It took a parody season and turned it into an After the End scenario where most of humanity has been slaughtered or enslaved. The show also pokes fun at PR conventions, Ziggy and Dr. K are awesomeness personified, there's a fight where the Rangers use a baby carriage against some mooks (WITH the baby inside), and a Scottish Ranger.
