Decade isn't even a "real" rider show, it's just a bunch of parodies of the previous shows. It's generally considered one of the weakest shows or something to watch once you've finished all else.
Kuuga is one of the most recommended starting points for Rider so you're well off starting with it. Just make sure you get Midnight Crew subtitles.
Now it is one of the older shows.
So expect 1) more practical, real effects instead of cheap CGI
2) when there is CGI however, it's terrible
3) It's much, much darker than you'd expect from a kids show in the west. Tons of people die every episode often in unpleasant, horrific ways especially in the really old shows.
4) Riders actually ride motorbikes to get around
Kuuga specifically features heavy police involvement also and a sort of buddy cop bromance between Kuuga and his cop friend. I think it's pretty cool personally. While in other shows the police just sort of... don't get involved or show up once or twice to give the hero trouble.
Kuuga is also one of the most lovabale Rider characters. That smile.
Have fun!