As far as Sentai goes, the shows started getting a bit more light hearted around the time of Carranger. I havent seen much before Gaoranger, but Ive seen some of Liveman, Fiveman, and Dairanger and those are pretty intense.
With that being said, dont let the "light-heartedness" steer you away from watching other shows like Gaoranger, Dekaranger, Magiranger. those shows are pretty light but they have very good storylines. Some say that Dekaranger doesnt have much of story, but the characters do develop throughout the show and its sort of a different take on the story format.
Since it seems like Go-onger is the first Sentai you were exposed to recently, its understandable that you have that impression, but, to me, its the oddball of a line of very good shows.
As far as Kamen Rider, anything from Kuuga up to the present is fair game for a good show. My favorite is Faiz, but I really, REALLY enjoyed every other shows Ive seen since Agito. Kuuga is in the process of being subbed I believe so if you want, you can start from Agito and go up or from Den-o and go down. Either way, you are in for a treat.