Jay:
I don't believe I've listened to any kpop song older than "Kiss - Because I'm a Girl", but I do know many singers that started in the 90s who had songs in the 2000s :
Shinhwa - Without You, Gonna be Alright
Lee Seung Chul - Fate, No One Else, Malri Flower
Fly to the Sky (?) - Missing You, My Angel, Question
I find that most ppl don't listen to anything before Big Bang's Haru Haru (2008), so even those feel really old already.
I dont listen to any 3rd gen idols such as exo and bts
well this is probs my 2nd fave song by him after fate: boohwal - never ending story (vocalist: lee seung chul)
but anyways theres a ton of amazing 90s singers such as:
Kim Min Jong (my fave actor and 2nd fave singer)
Lee Jung Hyun
Uhm Jung Hwa
Lee Hyun Woo
COOL (mostly just Lee Jae Hoon cuz hes just awesome)
Sechskies (even the rappers can sing well, especially eun ji won)
g.o.d
YB (Yoon Do Hyun)
yurisangja
roor'ra
Vibe (Yoon Min Soo debuted in the 90s with 4men)
early 2000's:
Sung Si Kyung (my fave singer, i listen to his songs whenever i can )