Great thread. Here is my contribution:

boom bap:
https://www.youtube.com/watch?v=21-GQfdanr8
https://www.youtube.com/watch?v=PgpDN-bARwc
https://www.youtube.com/watch?v=r7aasDfVAuw
https://www.youtube.com/watch?v=l5iGGLKad-I

alternative/underground:
https://www.youtube.com/watch?v=YA7USqZ52Mc
https://www.youtube.com/watch?v=z7hLhJtwuHI&list=PLqP4zxPUF84R5z4BEkSsyw6r5heel5wT8&index=3
https://www.youtube.com/watch?v=EyR66BrUpOc&list=PLqP4zxPUF84R5z4BEkSsyw6r5heel5wT8&index=9
https://www.youtube.com/watch?v=-p1KZCAfiyg

hungarian hardcore:
https://www.youtube.com/watch?v=ypZQTTBtpkA&list=PLDB4FAFFE323EA4D0&index=2
https://www.youtube.com/watch?v=mx7KGtyaCIg&list=PLDB4FAFFE323EA4D0&index=5

uk's finest:
https://www.youtube.com/watch?v=V-elPhIkU9U
https://www.youtube.com/watch?v=81PPp4ecUXE
https://www.youtube.com/watch?v=Rm5NuVacANE

sometimes I wonder how many underground artists from the 90s and 00s are out there to discover, perhaps never to be discovered.