Despite the flaws it had, I think RX could be a pretty fun show to start with if you are not ready for the whole mystery/drama theme that Kuuga started.
RX suffered from a personality split. It both wanted to be a KR show and a Metal Hero show, and it ended up getting a little bit from both franchises. You have the evil space-alien group, consisting of various generals who all have their own monster-types. You have a light saber wielding Rider who owns an intelligent super-car, and can transform into a robotic or a bionic form.
At the same time you have all the showa-Rider elements in there too, such as the cool bike-action.
RX also feels much less episodic than Black, since it keeps introducing more and more new things along the way, be it new powers or supporting characters. There are filler-eps, yes, but they are actually quite fun.
RX is also one of only Riders shows, were the Rider fights on other planets from time to time. This gives a little more variation to the fights.
So, if you like a more simple story with good fights, humor and classic villains, RX is the show for you, and a good springboard to the Heisei era.