The Seishin Series are worth checking out as everyone has already said.
[HIDE]I might have missed something, but are you sure it was Naoto and Ryouko at the end? [/HIDE]
Start with GS, then JR, Etc. The GS are somewhat related- if not for the joint Sazer X movie coming out...
For a synopsis of GS:
[HIDE]GS was based on the 12 houses of the Zodiac, with 4 tribes: Earth, Wind, Water, Fire - your basic Captain Planet. They're the ancestors of some ancient race which was wiped out eons ago. Various alien races come to try and re-wipe out Earth. The GS make a last stand to prevent history from repeating itself. There's a lot of subplots etc along the way. It has humor at times, but personally, I liked it. It gets slow a bit in the 30's, as do almost all shows, but then it picks up before and after that. I was pretty satisfied with the ending aside from one thing: a certain couple.[/HIDE]
For a synopsis of JR:
There's already been a pretty good synopsis given. I will say that the character relationships are fleshed out much better and fuller than GS. I liked both series; they each offer their own thing.
Sazer X: I can't comment too much having only seen the first 13 eps and didn't bother with the rest. I was turned off by the excess over-the-top humour, which speaks for itself. I may give it a second chance, but that's a pretty big if at this point. I also couldn't stand the designs. Still the fugliest seishin designs I've seen yet. I am told though by others who've stuck with it, that the story lines are better than GS and JR put together so who knows.