90's music is more of my time, and I find that 90's to be the golden age of KPOP, but others that grew up in different era would see this differently.
My rationalization is that 90's is when Korean audiences were introduced to Korean artists that performed Western style music (From the US). Like Seo Taiji for example, and much American style R&B and hip-hop style crept in during that time. In addition, there were musical style influenced from European house, and Japanese music as well. So, it was the turning point from the conservative Korean music to letting in outside cultural influences. So, this was big times for music to change. And there were so much experimentation because the industry had no idea how to go about things. There was no trainee system, etc.. They would just band together some talent and put some concept together to see if it sticks. Times like these are the experimentation times, and I find any golden age is just that. Just like 90's Japanese anime, which was a whole of taking risks with experimentation.
Also, you have to consider that of Korea transition to democracy in the late 80's, when they were coming out of conservative government party. They were a dictatorship that didn't condone freedom of art expression, so the 90's explosion had to happen.
I feel that there's a 2nd coming of experimentation with kpop. Once they find their own style, and then the experimentation arrises. I'm starting to see it shifting. It's much slower to take risks and experimentation when things become systematized.
Here's some classic music from the 90's, in which I find it to be the golden age of kpop.
Although Seo Taiji introduced American style music to the mainstream, he's not all that original if we see that he's just copying the concepts. He doesn't differentiate himself as an artist like by providing some idea that's of his own not borrowed. It's better not to copy too well because by not copying exactly you are bringing concepts fro your own culture, which results in a different product than an exactly copy. For originality and art sake, it's better IMO. It's not really art unless you contribute something from you.
Some were pretty wild and original like these groups below. I don't know where they got these concepts from