[Matrixbeast]

Ran and Retsu did draw the short straws for proper arcs, unfortunately, but it's just kind of the trade off that most Sentai's have. I know there are some that can give all the rangers the proper screen time they deserve, but those are the exceptions, rather than the rule. And in Gekiranger's case, I feel it's justified. They properly characterized and expanded on the villains, something rarely done in Sentai beyond one or two episodes.
Giving Rio and Mele such screen time was something entirely different for Sentai, and I enjoyed the wild change of pace. It was new and I felt it worked out well.
Gou's history with Rio tied him into the plot at large quite nicely, and Ken was just a blast to watch.

It's tough to balance 7 protagonists (Er, well..."Main characters" is probably more apt as 2 of them don't become protagonists until the last 5 episodes), and while there are places where they could've done it better, what they did receive came across well. Retsu growing to be more of a team player felt natural and Ran...Well, of the short sticks she drew the shortest, but she's just really likable~

It might be overhyped, but Gekiranger was actually the series that proved that "Yes, Sentai can actually be good."