Personally I say Kiva, I've never finished it, but am considering rewatching soon. Blade and Ryuki weren't my favorites either and to be fair, I've not actually seen any showa era ones. I've not seen any fansubs for them so if anyone knows where to find them that would be appreciated.
In my KR books there is no best or worst, There is good and bad, to be quite frank i would say Ryuki, But i wouldn't say its the worst season ever, just one i couldn't take a liking to.
I have to agree with this ( except for the Ryuki part, because I haven't seen it. ) The seasons that I've seen have their flaws and strengths, but the bad usually doesn't outweigh the good completely. Of course, that may change once I get to watching more and I finally find a KR season I dislike ...